Auto merge of #81956 - ssomers:btree_post_75200, r=Mark-Simulacrum

BTree: remove outdated traces of coercions

The introduction of `marker::ValMut` (#75200) meant iterators no longer see mutable keys but their code still pretends it does. And settle on the majority style `Some(unsafe {…})` over `unsafe { Some(…) }`.

r? `@Mark-Simulacrum`

diff --cc library/alloc/src/collections/btree/map.rs
Simple merge