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: 
My concern is a reader mistakenly thinking these ownership concepts are unique to heap memory. It may be enough to say somewhere below that it applies to stack memory too.