btree: add {Entry,VacantEntry}::insert_entry

[cuviper]

This matches the recently-stabilized methods on HashMap entries. I've
reused tracking issue #65225 for now, but we may want to split it.

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[ibraheemdev]

r? libs-api for new unstable API surface

[cuviper]

FWIW, I'm also working on a BTreeSet entry API to match HashSet's, and insert_entry will come in handy. I could still create and use that internally, but I don't see why we shouldn't expose it publicly as well.