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: 
That's not correct. The lifetime of such a struct is unrelated to 'a except that lifetime(struct) <= 'a. And references to such structs can again have different (shorter) lifetimes. Furthermore, saying "the lifetime" makes no sense since there can be more than one lifetime parameter.