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 was a bit confusing for me. I understand you're foreshadowing the Rust model here, but it seems like a there's an explanation missing.

```
Once we're done with the handle, we can't access the memory anymore, so we can't do anything useful for it. This means we should free the memory when we're done with the handle.
```

Or something like that.