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: 
Not just able: it is *responsible* for it. e.g. explicit ownership is one way Rust safely avoids memory leaks by ensuring each memory resource has exactly one thing responsible for cleaning it up (by default anyway, cf. `Rc`).