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 and the start of the next paragraph read a bit like "The ownership system has no cost! ... The ownership system has a cost!"  Perhaps something along the lines of "'zero-cost abstractions', in the sense that you pay as little as possible at *runtime*, in terms of execution speed, memory usage, or binary size", followed by "However, this runtime efficiency is gained in exchange for a learning curve."