Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
@netvl Thanks for the explanation!

I should have been more clear, I wasn't necessarily looking for an answer. I was trying to illustrate the areas I've been frustrated trying to learn about lifetimes as reasoning for why I think they should be explained in the guide. I'm very excited that lifetimes are being written about, but I was a bit bummed when I read through that explanation and learned nothing new. I definitely don't consider myself an expert with Rust (I spent a lot of time in IRC asking dumb questions), but I already had this basic understanding of lifetimes. I want a lot more detail because it's something I have to fight the compiler on, on a regular basis.

Explicit lifetimes are a bit weird. For example, where is the value of `'a` set? When you call `Foo::new()`? When trying to debug lifetimes in a large project, that seems like an important detail.