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.
One thing that's maybe *a tiny bit weird*: this talk about the number of allocations matching the number of deallocations... it kind of implies that it doesn't matter when or what you deallocate, so long as you deallocate as many times as you allocate.  I'm not sure how (or even if) you should rephrase this.

Also, it's just occurred to me that I don't remember seeing anything about use-after-free.  I had a quick look back up and couldn't see anything.