Type checking for type synonym families

Type checking for type synonym families

This patch introduces type checking for type families of which associated
type synonyms are a special case. E.g.

        type family Sum n m

        type instance Sum Zero n = n
        type instance Sum (Succ n) m = Succ (Sum n m)

where

        data Zero       -- empty type
        data Succ n     -- empty type

In addition we support equational constraints of the form:

        ty1 ~ ty2

(where ty1 and ty2 are arbitrary tau types) in any context where
type class constraints are already allowed, e.g.

        data Equals a b where
                Equals :: a ~ b => Equals a b

The above two syntactical extensions are disabled by default. Enable
with the -XTypeFamilies flag.

For further documentation about the patch, see:

        * the master plan
          http://hackage.haskell.org/trac/ghc/wiki/TypeFunctions

        * the user-level documentation
          http://haskell.org/haskellwiki/GHC/Indexed_types

The patch is mostly backwards compatible, except for:

        * Some error messages have been changed slightly.

        * Type checking of GADTs now requires a bit more type declarations:
          not only should the type of a GADT case scrutinee be given, but also
          that of any identifiers used in the branches and the return type.

Please report any unexpected behavior and incomprehensible error message
for existing code.

Contributors (code and/or ideas):
        Tom Schrijvers
        Manuel Chakravarty
        Simon Peyton-Jones
        Martin Sulzmann
with special thanks to Roman Leshchinskiy