Add function equality instance for finite functions to base

If the input type is bounded and enumerable and the result type can be compared for equality, functions can be checked for equality. Basically this instance:

instance (Bounded a, Enum a, Eq b) => Eq (a -> b) where
  f == g = all (\x -> f x == g x) [ minBound..maxBound ]

I often work with functions with finite domains and find [(a,b)] awkward. I thought this is generic enough that it might belong to base.

I'd also submit a patch, but I couldn't quite figure out where it should live. GHC.Base doesn't have Enum and Bounded; Enum is not where -> or Eq are declared; Eq is not declared anywhere; and Data.Function is only for combinators.

Trac metadata

Trac field	Value
Version	8.4.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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