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 don't know Rust, but this bit seems confusing. You seem to imply that `x` in the example below is a pointer and it doesn't look like it in the code below.

I'm not sure how to rephrase it best if that's not the case...