Optimise even and odd?

[benhutchison]

Just something to consider..

even and odd are defined generically as combinators rather than as typeclass methods.

even :: (P.Eq a, Integral a) => a -> P.Bool
even n = n `rem` (one + one) P.== zero

However, for most hardware-backed Integral types Int, Int64 etc there will be a likely much more efficient implementation by bit testing the lowest bit in the register.

If these were overridable on the Integral class, it might permit this specialization.