Move Away from Typed Racket #68

We tried using typed-racket, but the cost of ~1hr type-check times and the lack of full featured racket outweighs the benefit of types.

Here is a plan for moving away from typed-racket:

• stop using type checking:
  • remove uses of define-predicate
  • rewrite all uses of #lang typed/racket -> #lang typed/racket/no-check
  • debug this until it works again
  • remove all uses of Typed Racket's cast function.
• rewrite all language grammars as contracts
• change all passes to check these contracts
• find way to toggle contract checking for debug/production
• add facilities to improve writing of unit tests, such as those found in convert-closures.rkt
• lazily write unit tests for each pass as we edit them / discover bugs in the future
  • add to coding style
  • add to pull request check list
  • start using coverage checker