Fix some lingering :exit/:exit-code confusion
Allow whitespace around = and :=
Don't shadow fn. It's just so confusing.
Make more use of column and line number in errors.

Fixes #55.
Make throw-parse-error more convenient/pretty to use
Merge branch 'develop' of https://github.com/Factual/drake into develop

* 'develop' of https://github.com/Factual/drake:
  Make throw-parse-error more convenient/pretty to use
  Make more use of column and line number in errors.
  Don't shadow fn. It's just so confusing.
  Allow whitespace around = and :=
  Fix some lingering :exit/:exit-code confusion