Implement NFData for TypeRep in older GHC's

[ndmitchell]

NFData for TypeRep is only implemented in newer GHC's. I suggest implementing it as rnf . show in those older GHCs so at least it is available, even if less efficient.

[RyanGlScott]

I'm not sure if show would be sufficient to actually reduce these to normal form. As one example, on base-4.7, the Show instance for TyCon is simply:

instance Show TyCon where
  showsPrec _ t = showString (tyConName t)

So it will only force the tyConName, and not the tyConPackage or tyConModule.

[ndmitchell]

Yep, show might not be the right way forward, but you can do it by poking inside TyCon, which is entirely exposed via the Internal module in base-4.7.0.2

[RyanGlScott]

Ah! Good point. This is actually quite doable then.

#40