The unifying theme for this commit is weak, admittedly. I put together a
list of "expensive" functions when I originally proposed this whole
effort, but nobody's cared about that criterion. Still, it's a decent
way to bite off a not-too-big chunk of work.
Given the grab bag nature of this commit, the messages I used vary quite
a bit.
These methods could be misconstrued as modifying their arguments instead
of returning new values.
Where possible I made the note recommend a method that does mutate in
place.
