Factor out `NFData` constraint

[ulysses4ever]

Take for instance, DpsMergeSort4Par.hs. It has to repeat several times this ugly CPP:

#ifdef MUTABLE_ARRAYS
msortInplace :: (Show a, HasPrimOrd a, NFData a) =>
#else
msortInplace :: (Show a, HasPrimOrd a) =>
#endif

What it should do is to reuse the approach that allowed us to define HasPrim and HasPrimOrd, so that the result is simply:

msortInplace :: (Show a, HasPrimOrd a, HasNFData a) =>