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: 
There's two ways to think of `struct A<'b> { c: &'b str }`:

- `A` is valid as long as `c`
- `c` must be valid for at least as long as `A`

They both make total sense, and are equivalent, but sometimes it makes sense to consider one interpretation specifically (e.g. if you're writing a 'borrowing' wrapper that is mostly a means-to-an-ends the first makes sense, but if your struct is "valuable" in its own right, then maybe the second makes sense). The first is the most common though.

(PS. this sentence has an extra 'expect' or 'must' in it?)