Auto merge of #87696 - ssomers:btree_lazy_iterator_cleanup, r=Mark-Simulacrum

Auto merge of #87696 - ssomers:btree_lazy_iterator_cleanup, r=Mark-Simulacrum

BTree: merge the complication introduced by #81486 and #86031

Also:
- Deallocate the last few tree nodes as soon as an `into_iter` iterator steps beyond the end, instead of waiting around for the drop of the iterator (just to share more code).
- Symmetric code for backward iteration.
- Mark unsafe the methods on dying handles, modelling dying handles after raw pointers: it's the caller's responsibility to use them safely.

r? `@Mark-Simulacrum`