library-level docs for collections

Adds a high-level discussion of "what collection should you use for what", as well as some general discussion of correct/efficient usage of the capacity, iterator, and entry APIs.

Still building docs to confirm this renders right and the examples are good, but the content can be reviewed now.
OK, I see what you have in mind. I think the "temporarily" bit threw me off -- the idea here seems to be that you just have a pile of items, where you don't care about the order or any notion of key, or equality, or priority. And yes, that argues for `Vec` :)