Warn/error on top level symbols that are unused

It would be helpful, in the goal of maintaining code (e.g. deleting dead code), if DSLX had a warning/error that would:

1. Internally note all symbols (e.g. constants, functions) that are public, used in a test, or indirectly referenced via one of the former (e.g. used by a public function).
1. Warn/error on all symbols that are not directly or indirectly used.