Add doesNotContainKey/s to Guava Multimap asserts

Behavior matches the asserts for regular java.util Map asserts in core.

Thanks @ryber, we will review your PR shortly !

Thanks @ryber, first round of code review done, mostly minor stuff ;-)

@joel-costigliola feedback pushed up on its own commit. If you would rather I squash them and force push to the branch I can, or you can squash on merge, or you can leave them as different commits of course. I was not sure what your preference was. I think I got everything 😄

We usually squash the commits into one to make logical commits, cool if you do it but no problem if you let us

thanks @scordio updated with feedback applied

Thanks for your first contribution, @ryber!

We'll go ahead with the merge after releasing 3.25.2 (expected later this month).

@scordio oh sorry! I didn't catch that you suggested a name change. I went ahead and changed it

Thanks for your first contribution, @ryber!