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.
I believe that it should be `*num += 1` and `num: Box<int>` should be `mut num: Box<int>`. Otherwise this piece does not compile.