instance inference

[Ross Paterson]

As a termination test, how about no restrictions on context and head
except:

   Each assertion in the context must satisfy
    * the variables of the assertion are a sub-multiset of those of the
      head (though they may be the same), and
    * the assertion has fewer type constructors and variables (taken
      together and counting repetitions) than the head.

This ensures that under any ground substitution the context assertion has
fewer type constructors than the head.

It would accept all instances accepted by the current test, plus

	instance C a
	instance C (s a) => C (Sized s a)
	instance (C1 a, C2 b) => C a b
	instance C1 Int a => C2 Bool [a]
	instance C1 Int a => C2 [a] b
	instance C a a => C [a] [a]

But not things like

	instance (C1 a, C2 a, C3 a) => C a

because that would need to check other instances as well.