Auto merge of #78631 - ssomers:btree-alias_for_underfull, r=Mark-Simulacrum

BTreeMap: fix pointer provenance rules in underfullness

Continuing on #78480, and for readability, and possibly for performance: avoid aliasing when handling underfull nodes, and consolidate the code doing that. In particular:
- Avoid the rather explicit aliasing for internal nodes in `remove_kv_tracking`.
- Climb down to the root to handle underfull nodes using a reborrowed handle, rather than one copied with `ptr::read`, before resuming on the leaf level.
- Integrate the code tracking leaf edge position into the functions performing changes, rather than bolting it on.

r? `@Mark-Simulacrum`