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: 
I think it's better to not call the period-of-validity-of-an-object a lifetime, since that is needlessly confusing with lifetimes of references/named lifetimes. I saw someone propose "life cycle" for the first thing.