deepseq: instance NFData (a -> b)

[Henning Thielemann]

According to Haddock comments, between deepseq-1.2 and deepseq-1.3 an 
instance for NFData on functions was introduced without previous 
discussion. I find this instance pretty problematic since it has no 
superclasses. The correct instance would be certainly something like

instance (Enumerate a, NFData b) => NFData (a -> b)

where Enumerate would be a new class that allows to enumerate all values 
of a type. This would be hardly useful because it is pretty inefficient. 
I'd prefer that the instance is removed, again, or even better, be 
replaced by a non-implementable instance. Alternatively we should replace 
it by a correct implementation with corresponding superclasses. If we do 
the second, then we could still omit the Enumerate instance for types 
where enumeration of all values of the type is too expensive.

I assume that the instance was added to simplify automatic derivation of 
NFData instances. However, I think it would be better if people insert 
custom deepseq implementation for the expected functions then.