Rollup merge of rust-lang#101727 - est31:stabilize_map_first_last, r=m-ou-se

Stabilize map_first_last

Stabilizes the following functions:

```Rust
impl<T> BTreeSet<T> {
    pub fn first(&self) -> Option<&T> where T: Ord;
    pub fn last(&self) -> Option<&T> where T: Ord;
    pub fn pop_first(&mut self) -> Option<T> where T: Ord;
    pub fn pop_last(&mut self) -> Option<T> where T: Ord;
}

impl<K, V> BTreeMap<K, V> {
    pub fn first_key_value(&self) -> Option<(&K, &V)> where K: Ord;
    pub fn last_key_value(&self) -> Option<(&K, &V)> where K: Ord;
    pub fn first_entry(&mut self) -> Option<OccupiedEntry<'_, K, V>> where K: Ord;
    pub fn last_entry(&mut self) -> Option<OccupiedEntry<'_, K, V>> where K: Ord;
    pub fn pop_first(&mut self) -> Option<(K, V)> where K: Ord;
    pub fn pop_last(&mut self) -> Option<(K, V)> where K: Ord;
}
```

Closes rust-lang#62924

~~Blocked on the [FCP](rust-lang#62924 (comment)) finishing.~~ Edit: It finished!

Author: matthiaskrgr

src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(rustc_private)]
-#![feature(map_first_last)]
 #![feature(map_try_insert)]
 #![feature(never_type)]
 #![feature(try_blocks)]