Entry::and_modify should use FnOnce

[Zoxc]

This should change to use FnOnce instead of FnMut before it lands on stable.
https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html#method.and_modify

cc @rust-lang/libs

[alexcrichton]

Labelling as a regression to ensure we keep track of this, although it's not technically a regression

[LunaBorowska]

Now that this is fixed, should it be marked as regression-from-stable-to-beta. It's technically not a regression, it's was marked as such for tracking purposes.