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: 
unless you use `Rc`, `Arc` or build other resource sharing abstractions yourself. I'm also pretty sure a channel's buffer (a kind of resource) is owned by two things usually named `tx` and `rx`. So, maybe you should start this paragraph in a less restrictive way and say something like "usually we deal with unique ownership relations" (out of convenience, simplicity, zero runtime overhead, mutability of the "ownee" etc). Anyhow, the reader should learn that the ownership relations are always clearly defined and what it means to own something.