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.
> is alive for *at most* the time the reference to an `int` is

It's fine to have `Foo` live less. Also, `Foo` *values* matter, not just references to them.

Maybe:

> We need to ensure that any `Foo` cannot outlive the reference to an `int` it contains.