Tree.parent and Symbol.owner

How are we going to manage to pull this off?

Read the green vs red tree write-up about Roslyn + keep in mind that in
the new vision macros expand after typer, so we can be sure that parents
are consistent.

What about whitebox macros you ask? Well, there's another cute vision.
Whitebox macros = blackbox macros + fundep materializers, and fundep
materializers only need types not trees, and we actually don't have
to maintain consistency of any parents / expose any definitions other
than the types passed to the macro.