Add `HashSet::get_or_insert_with_mut`#396

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yyny wants to merge 2 commits intorust-lang:masterfrom

Commits on Feb 14, 2023

Add `HashSet::get_or_insert_with_mut`
yyny
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Commits on Feb 16, 2023

Document the expected invariants for `get_or_insert_with` and `get_or_insert_with_mut`
yyny
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