Generalize injective type families

With injective type families we can now declare that type family result determines the arguments. But ITFs are not yet as expressive as functional dependencies. For example with FDs I can say:

data Nat = Zero | Succ a

class Add a b r | a b -> r, r a -> b
instance                Add Zero     b b
instance (Add a b r) => Add (Succ a) b (Succ r)

Here we declare that the result and the first argument taken together uniquely determine the second argument. This is not currently possible with ITFs:

type family AddTF a b = r | r a -> b where
     AddTF Zero     b = b
     AddTF (Succ a) b = Succ (AddTF a b)

But we want to be able to say that.

Trac metadata

Trac field	Value
Version	7.11
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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