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: 
Is it confusing to start with just a box/heap example? In theory I like the idea of also showing and narrating a stack example, but trying to explain them both may be too unwieldy.