Note [Type synonym families]

* Type synonym families, also known as "type functions", map directly
  onto the type functions in FC:

        type family F a :: Type
        type instance F Int = Bool
        ..etc...

* Reply "yes" to isTypeFamilyTyCon, and isFamilyTyCon

* From the user's point of view (F Int) and Bool are simply
  equivalent types.

* A Haskell 98 type synonym is a degenerate form of a type synonym
  family.

* Type functions can't appear in the LHS of a type function:
        type instance F (F Int) = ...   -- BAD!

* Translation of type family decl:
        type family F a :: Type
  translates to
    a FamilyTyCon 'F', whose FamTyConFlav is OpenSynFamilyTyCon

        type family G a :: Type where
          G Int = Bool
          G Bool = Char
          G a = ()
  translates to
    a FamilyTyCon 'G', whose FamTyConFlav is ClosedSynFamilyTyCon, with the
    appropriate CoAxiom representing the equations

We also support injective type families -- see Note [Injective type families]