Get rid of extensions:

This branch is for a `Haskell98'-like version of the BlueJelly language.
Therefore I've removed the extensions planned in the main branch, namely:

  - Functional dependencies (so classes won't be multi-parameter)
  - Rank-n (n > 1) polymorphism
  - Typed pattern bindings
  - Existencial types, polymorphic/qualified datacons
  - Anonymous type variables
  - Variable-only patterns in records