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 felt strange when I read "This code compiles." here, because I think "a programmer compiles code." "This code works" or "This code can be compiled" would be natural for me. What do you think?