New ownership guide

This is the start of the new "Ownership" guide, to replace the lifetimes guide.

I'm still working on it, but I wanted to share my progress, as this guide is _very_ important, and I'd like as many eyes on it as possible. :smile: 
Also, there's two concepts here:

- lifetimes of references
- named lifetimes

The first is the general "this reference is valid" "this reference is not valid" idea, the second is how a Rust programmer describes the validity-relationships between references.