update liquidhaskell to use ghc7.6

This is a sizable patch, but not very interesting. Most of the diff is in the Desugar modules, which have just been patched to match GHC's modules.

The noteworthy changes are:

- `GhcInterface.hs`: I updated our dynFlags to set `HscInterpreted` and `LinkInMemory` in order to give us access to *all* top-level defs in the target module.

- `RefType.hs`: I updated `freeTyVars` to ignore the tyvars that occur in class constraints, which results in an ordering that seems to match GHC.

Note: the branch is called ghc762, but I tested on 7.6.2 and 7.6.1, it works for both versions.