Work on removing units

This removes all the units from the core (ie, `racket/collects`). Once this is done, we'll see about moving `racket/unit` itself to a new package.

Things done:
- [x] Move `dynext` units to `compiler-lib`.
- [x] Made other units really wrappers around modules.
- [x] Make `setup-unit` (now called `setup-core`) a function that does the work.

Things still to do: 
- [x] Run a full set of tests.
- [x] Figure out if setup-options are ok as a module.
- [x] Figure out what exactly is needed for backwards compatibility.
- [x] Implement the above.
- [x] Rename the `setup@` binding.
- [x] Decide about moving `dynext/file` back to core (currently has small amount of code duplication).