Crystal is now community supported on Travis CI

I did not separate types.cr to type.cr and expr.cr because function type
requires the definition of Crisp::Expr.  They depend on each
definitions.

- some compilation error
- STDERR seems not to work in Crystal 0.7.2

- argument of Crisp::Interpreter#initialize
- fix typo: Priter -> Printer
- fix Crisp::Printer#print
- add missing `extend self`