Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
This isn't really a conventional lifetime elision situation. Lifetime elision usually refers to implicitly connecting the lifetimes of input references with output references, not just implicitly inserting anonymous lifetimes.