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: 
Should this be more forceful? "to tell the compiler that `a_string` **will** be valid as long"...

Using **expect** makes it sounds like it would be nice if the compiler kept it around but we wouldn't worry much about it either way.
(I expect it probably is more forceful/strict but I'm not sure which is why I'm asking)