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: 
You seem to be conflating two kinds of lifetimes here: The lifetime of a reference and the lifetime of a reference's pointee. It's true, that lifetime parameters only tend to appear (in source code) in combination with references, but I would try to define lifetimes without mentioning references. Every value/object has a lifetime. It usually refers to some scope or 'static in case the lifetime matches the duration of the program. And then: If one starts referring to other things via references, we better make sure that the pointee stays alive for at least as long as the reference exists. To make sure that this is the case, lifetimes are part of the type system: lifetime parameters are used to "decorate" reference types (even if deduced and invisible).