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.
This ASCII art might look nicer as

```
-+ x comes into scope
 |
 |
---+ y comes into scope
 | | error here
---+ f & y go out of scope
 |
 |
-+ x goes out of scope
```

i.e. attempt to treat the y scope as building on top of the x scope.