Convert to (somewhat) idiomatic Racket

  - use modules and provide/require
  - use module+ and rackunit for testing
  - `raco test mk-rd.scm` to run tests
Use .rkt extension
Actually provide things from mk-rd.rkt
Uses `findf` instead of `exists`
More idiomatic racket code.

Better definitions for `remp` and `datum->string`.
Merge pull request #2 from takikawa/master

Start porting Racket version to Racket style