Remove GHC-specific bits from liquid-fixpoint

[facundominguez]

At the moment the extensionality feature hardcodes the datatype definition of pairs.

Pairs are produced by liquidhaskell, but their definition is not made explicit in the fixpoint queries.

This GHC-specific definition should be removed from liquid-fixpoint (this part is easy).

Then liquidhaskell should define a type of pairs that liquid-fixpoint can handle. This will require some investigation to see if LH has a datatype representation of tuples and if so why it isn't appearing in fixpoint queries.