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.
this could be qualified to "many abstractions", rather than implying that all abstractions cost as little as possible (it's trivially possible to write abstractions that aren't as cheap as possible in Rust; people can and do take statements of "zero cost abstractions" to mean "every abstraction is a ZCA").