Auto merge of #79521 - ssomers:btree_cleanup_2, r=Mark-Simulacrum

BTreeMap: relax the explicit borrow rule to make code shorter and safer

Expressions like `.reborrow_mut().into_len_mut()` are annoyingly long, and kind of dangerous for the reason `reborrow_mut()` is unsafe. By relaxing the single rule, we no longer have to make an exception for functions with a `borrow` name and functions like `as_leaf_mut`. This is largely restoring the declaration style of the btree::node API about a year ago, but with more explanation and consistency.

r? `@Mark-Simulacrum`

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