Freelist defrag

[uint]

Closes #990

This PR picks up where #992 left off (thanks @askoa!). The implementation is mostly the same except:

• FreeList::defrag does not interact with Vector internals (see comments in feat: defrag function for FreeList and UnorderedMap #992)
• Vector gets the new remove_tail method to support the above in a slightly more efficient fashion
• the defrag test does not assert stuff irrelevant to the feature - those assertions are instead pulled into new, more atomic tests (new_bucket_is_empty, occupied_count_gets_updated)

[frol]

@uint Thanks for the contribution! This PR is good to go once those two comments I left are addressed

[frol]

@austinabell If you would have the capacity, it would be great to hear your input here.

[austinabell]

I don't have bandwidth to review in depth right now, but shared some thoughts from skimming the PR. Hope it's helpful! :)

[uint]

I know this is horribly out of scope, but cargo doc kept complaining about broken links in doc comments and it was an easy fix, so I added that. Looks like these were mostly the result of making UnorderedSet iterator types public recently.

[frol]

@austinabell Thanks for the review! I feel @uint addressed all of them, and I am ready to merge this PR once all tests pass.