Rollup merge of rust-lang#56648 - RalfJung:btree, r=sfackler

Fix BTreeMap UB

BTreeMap currently causes UB by created a shared reference to a too-small allocation.  This PR fixes that by introducing a `NodeHeader` type and using that until we really need access to the key/value arrays.  Avoiding run-time checks in `into_key_slice` was somewhat tricky, see the comments embedded in the code.

I also adjusted `as_leaf_mut` to return a raw pointer, because creating a mutable reference asserts that there are no aliases to the pointee, but that's not always correct: We use `as_leaf_mut` twice to create two mutable slices for keys and values; the second call overlaps with the first slice and hence is not a unique pointer.

Fixes rust-lang#54957

Cc @nikomatsakis @gankro