feat: implement entry for BTreeMap similar to std lib

[hpeebles]

This allows you to lookup an entry by searching the map, then modify the value and insert it back into the map without having to search again.

The API is similar to entry on the std lib BTreeMap, but is not able to return mutable references, so you need to either use and_modify or get the owned value, update it, then insert it back in the entry.

For example in the std lib you would write this

match map.entry(1) {
    Occupied(mut e) => {
        let value = e.get_mut();
        *value += 1;
    }
    Vacant(e) => {
        e.insert(1);
    }
}

Or using the shorthand syntax

map.entry(1).and_modify(|v| *v += 1).or_insert(1)

Whereas using this new API for StableBTreeMap you would write it as this

match map.entry(1) {
    Occupied(e) => {
        let value = e.get();
        e.insert(value + 1);
    }
    Vacant(e) => {
        e.insert(1);
    }
}

Or using the shorthand syntax

map.entry(1).and_modify(|v| *v += 1).or_insert(1)

I've added some benchmarks which compare using get then insert vs using entry for 10k u32 values, from the results you can see that in this scenario using entry is roughly 37% faster

| status | name                            | calls |     ins |  ins Δ% | HI |  HI Δ% | SMI |  SMI Δ% |
|--------|---------------------------------|-------|---------|---------|----|--------|-----|---------|
|  new   | btreemap_get_and_incr           |       | 597.30M |         |  0 |        |   0 |         |
|  new   | btreemap_get_and_incr_via_entry |       | 374.19M |         |  0 |        |   0 |         |

[github-actions]

canbench 🏋 (dir: ./benchmarks/nns) 3a2bfab 2026-04-01 09:52:41 UTC

✅ ./benchmarks/nns/canbench_results.yml is up to date
📦 canbench_results_nns.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./benchmarks/io_chunks) 3a2bfab 2026-04-01 09:53:11 UTC

✅ ./benchmarks/io_chunks/canbench_results.yml is up to date
📦 canbench_results_io_chunks.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 18 | regressed 0 | improved 0 | new 0 | unchanged 18]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 18 | regressed 0 | improved 0 | new 0 | unchanged 18]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 18 | regressed 0 | improved 0 | new 0 | unchanged 18]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./benchmarks/vec) 3a2bfab 2026-04-01 09:52:29 UTC

✅ ./benchmarks/vec/canbench_results.yml is up to date
📦 canbench_results_vec.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 16 | regressed 0 | improved 0 | new 0 | unchanged 16]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./benchmarks/memory_manager) 3a2bfab 2026-04-01 09:52:23 UTC

✅ ./benchmarks/memory_manager/canbench_results.yml is up to date
📦 canbench_results_memory-manager.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 3 | regressed 0 | improved 0 | new 0 | unchanged 3]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./benchmarks/btreeset) 3a2bfab 2026-04-01 09:52:40 UTC

✅ ./benchmarks/btreeset/canbench_results.yml is up to date
📦 canbench_results_btreeset.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   No significant changes 👍
    counts:   [total 100 | regressed 0 | improved 0 | new 0 | unchanged 100]
    change:   [max +931.32K | p75 0 | median 0 | p25 0 | min 0]
    change %: [max +0.13% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  heap_increase:
    status:   No significant changes 👍
    counts:   [total 100 | regressed 0 | improved 0 | new 0 | unchanged 100]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   No significant changes 👍
    counts:   [total 100 | regressed 0 | improved 0 | new 0 | unchanged 100]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------
CSV results saved to canbench_results.csv

[github-actions]

canbench 🏋 (dir: ./benchmarks/btreemap) 3a2bfab 2026-04-01 09:53:47 UTC

❌ ./benchmarks/btreemap/canbench_results.yml is not up to date
If the performance change is expected, run canbench --persist [--csv] to update the benchmark results.
📦 canbench_results_btreemap.csv available in artifacts

---------------------------------------------------

Summary:
  instructions:
    status:   New benchmarks added ➕
    counts:   [total 231 | regressed 0 | improved 0 | new 2 | unchanged 229]
    change:   [max +31.33M | p75 +2.24M | median +19.88K | p25 0 | min -8.34M]
    change %: [max +0.52% | p75 +0.29% | median 0.00% | p25 0.00% | min -0.34%]

  heap_increase:
    status:   New benchmarks added ➕
    counts:   [total 231 | regressed 0 | improved 0 | new 2 | unchanged 229]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

  stable_memory_increase:
    status:   New benchmarks added ➕
    counts:   [total 231 | regressed 0 | improved 0 | new 2 | unchanged 229]
    change:   [max 0 | p75 0 | median 0 | p25 0 | min 0]
    change %: [max 0.00% | p75 0.00% | median 0.00% | p25 0.00% | min 0.00%]

---------------------------------------------------

Only significant changes:
| status | name                            | calls |     ins |  ins Δ% | HI |  HI Δ% | SMI |  SMI Δ% |
|--------|---------------------------------|-------|---------|---------|----|--------|-----|---------|
|  new   | btreemap_get_and_incr           |       | 542.98M |         |  0 |        |   0 |         |
|  new   | btreemap_get_and_incr_via_entry |       | 374.87M |         |  0 |        |   0 |         |

ins = instructions, HI = heap_increase, SMI = stable_memory_increase, Δ% = percent change

---------------------------------------------------
CSV results saved to canbench_results.csv