Refactor the `Instance` datatype to use non-closed `SType`s

[jorisdral]

#1247 changes the Instance type in the HsBindgen.Backend.SHs.AST module to include superclasses, and we add a TFree constructor to the SType datatype in the same module to represent free type variables. This allows us to write instances like:

instance ( ty ~ CFieldType MyInt "un_MyInt"
         ) => HasField "un_MyInt" (Ptr MyInt) (Ptr ty) where

The downside of this approach is that ty is not really a free type variable. ty is bound implicitly in the class instance. For example, the following are equivalent:

instance S ty => C ty
instance forall ty. S ty => C ty

So, using TFree is kind of cheating. If we wanted to represent instance types correctly, we should refactor Instance to use non-closed types with properly scoped type variables instead. And then we can probably remove TFree altogether.