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.
As @jaredonline says, there's another tricky use of `'static`. A bound satisfied by things that *could* live forever (they don't necessarily actually live forever, but it's not illegal to make them do so), that is, equivalently:

- have no scope restrictions (i.e. can be returned from/stored outside any scope)
- contain no non-`'static` references
- have ownership of their contents

(Those are all essentially equivalent formulations.)

E.g. given `fn foo<T: 'static>() {}` then `foo::<int>()`, `foo::<Box<int>>()` and `foo::<&'static int>()` are all OK, but `foo::<Foo<'a>>()` and `foo::<&'a int>()` are not.

This falls under the general topic of lifetime bounds, `fn bar<'a, T: 'a>()`, but `'static` is somewhat special, since it covers all types that contain no lifetimes at all.