Collapse to a single crate
Merge pull request #1 from kemurphy/single-crate

Collapse to a single crate
Switch to new regexp library
Merge pull request #2 from kemurphy/upstream-regexp

Switch to new regexp library
Clean up lexer test
Make the lexer into an iterator
Fix type parameters; update tests
Change some token names
Basic skeleton of a parser for expressions.
Add a few more comments.