feat!: add lazy BTreeMap iterator for improved performance (breaking API change) (#375)

This PR changes the `BTreeMap` iterator's `Item` type from `(K, V)` to
`LazyEntry` to enable lazy evaluation of keys and values.

This is a **BREAKING API CHANGE**, as it deviates from the canonical
`(K, V)` pattern of `std::collections::BTreeMap`.

The key motivation is **performance**: previously, each `next()` call
eagerly deserialized and cloned key and value, even if the consumer
discarded them. With `LazyEntry`, key and value access is deferred until
explicitly requested, significantly reducing unnecessary allocations and
cloning in cases where only part of the entry is needed or iteration is
short-circuited.

Code change summary:
- Introduced `LazyEntry` with on-demand access to key/value.
- Replaced `Box<Node<K>>` with `Rc<Node<K>>` to allow shared ownership
between iterator and entries.
- Updated all internal iterator consumers to explicitly call
`.into_pair()` where eager materialization is needed.

Performance change summary:
- `BTreeSet`: 68 improvements, no regressions. Median instruction count
dropped by **-6.61%**, best case **-17.24%**
- `BTreeMap`: 1 small regression (+2.09%), 10 improvements, with up to
**-98.33%** fewer instructions in range-heavy cases

Author: maksymar

benchmarks/btreemap/canbench_results.yml

@@ -777,7 +777,7 @@ fn range_key_sum_helper_v2(count: usize, size: usize) -> BenchResult {
     bench_fn(|| {
         btree
             .range((Bound::Included(0), Bound::Included(size as u32)))
-            .map(|(k, _)| k)
+            .map(|entry| *entry.key())
             .sum::<u32>()
     })
 }
@@ -794,8 +794,8 @@ fn range_value_sum_helper_v2(count: usize, size: usize) -> BenchResult {
     bench_fn(|| {
         btree
             .range((Bound::Included(0), Bound::Included(size as u32)))
-            .filter(|(k, _)| k % 3 == 0)
-            .map(|(_, v)| v.len())
+            .filter(|entry| *entry.key() % 3 == 0)
+            .map(|entry| entry.value().len())
             .sum::<usize>()
     })
 }