Auto merge of #86520 - ssomers:btree_iterators_checked_unwrap, r=Mark-Simulacrum
Auto merge of #86520 - ssomers:btree_iterators_checked_unwrap, r=Mark-Simulacrum

BTree: consistently avoid unwrap_unchecked in iterators

Some iterator support functions named `_unchecked` internally use `unwrap`, some use `unwrap_unchecked`. This PR tries settling on `unwrap`. #86195 went up the same road but travelled way further and doesn't seem successful.

r? `@Mark-Simulacrum`

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust