overhaul &mut suggestions in borrowck errors

[fee1-dead]

• This refactors the logic so that it does not use fuzzy string matching for suggestions; it instead uses information directly from MIR.
• If something comes from a custom Index impl for which the IndexMut trait does not apply, do not suggest adding mut after &.
• Suggest get_mut with unwrap if error is fired on BTreeMap or HashMap.

Supersedes #144018 cc @xizheyin
Closes #143732

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