Stabilize `btree_entry_insert` feature
This stabilises `btree_map::VacantEntry::insert_entry` and `btree_map::Entry::insert_entry`, following the FCP in [tracking issue](https://github.com/rust-lang/rust/issues/65225).
New stable API:
```rust
impl<'a, K: Ord, V, A: Allocator + Clone> Entry<'a, K, V, A> {
pub fn insert_entry(self, value: V) -> OccupiedEntry<'a, K, V, A>;
}
impl<'a, K: Ord, V, A: Allocator + Clone> VacantEntry<'a, K, V, A> {
pub fn insert_entry(mut self, value: V) -> OccupiedEntry<'a, K, V, A>;
}
```
(FCP ended almost a year ago, so if it's needed for process we could rerun it)
Closes: https://github.com/rust-lang/rust/issues/65225
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