Move info into individual modules.
Remove container guide.

This isn't really what guides are for, this information belongs in the
module-level docs.

Fixes #9314.
auto merge of #16887 : steveklabnik/rust/guide_iterators, r=brson

This isn't ready to merge yet.

The 'containers and iterators' guide is basically just a collection of stuff that should be in the module definitions. So I'm moving the guide to just an 'iterators' guide, and moved the info that was there into the right places.

So, is this a good path forward, and is all of the information still correct?