[ava-cassiopeia]

This is a convenience on top of IgnoreMapEntries() that allows for uses like:

mapA := map[string]string{ /* ... */ }
mapB := map[string]string{ /* ... */ }

diff := cmp.Diff(mapA, mapB, cmpopts.IgnoreMapKeys("tobeignored", "another"))

I've wanted this now a few times while writing tests for work, and I figured I'd see what y'all think about adding this to the actual cmpopts package so I can avoid adding basically this same implementation to a common lib for my team's projects.

Some misc comments:

• I added a copyright comment to the top of the new test file, which I guessed should have the current year.
• I can add more tests for more types of comparables, if desired.
• This uses generics, which are somewhat new, but I see other uses of newer go features here so hopefully that's ok? It could probably be rewritten without generics if needed for backwards compatibility, though.

[ava-cassiopeia]

Hmm, the test failure here seems unrelated to this change. Maybe something with the version of go that the macos check is pulling in..?