constants module complete, add tests
  * add moderately ok set of exunit tests for Constants and Debug
  * Quaff.Constants into usable state:
    * make compilation errors readable (not all cases handled yet)
    * greatly improved include-file finding
    * switch to parse_exprs + erl_eval:exprs for getting constants vals
      this allows using things like 60*60*24 in a constant
      but exprs that evaluate to functions must be excluded