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.
Lifetimes and borrowing are zero-cost in the sense that they're a compile-time check. On the other hand, the ownership system does have a runtime presence. Both systems are often barriers to writing optimal code because they forbid many correct patterns that can't be proven correct. You do pay a runtime cost for these features, but it's predictable and can be avoided with `unsafe` when it's important.