Put the whole language in a single file, with submodules.

Also makes "#lang rash" work, assuming proper "raco link".
Update README.
Proper test suite.
Have rash commands output lists of strings.

More useful when programmatically using the output.

Also, add simple pipe support.
Add slightly larger example.

Showcases how calling rash from Racket just works.
Add example where Racket code is called from a rash pipeline.
Turn rash into a package