Remove meaningless/bad Token.SPECIAL category.
Change character sets to explicit sets.

This probably improves performance, but more importantly, it's clear that some
of the Chars.* constants are character sets, and some are individual
characters.
Add tokenize_string function.
Add very minimal compiler. Supports:

  - Multiple-command notation (e.g. 5+ for +++++)
  - Compile from input file to stdout.

Does not support:

  - Any other language features.
  - Any other in/out combinations.