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.
It *might* be worth as an aside, again for higher-level programmers, to mention *why* string literals are `'static`: they're baked into the compiled binary's data segment.  The actual `&str` value is just providing a slice of that data segment, and since the data segment is only unloaded when the program itself exits... `'static` lifetime.