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.
Moving data into and out of functions is a useful way of doing in-place mutation for efficiency whilst maintaining a pure, referentially transparent API. I wonder if this could be addressed in a future section.