Remove Unbox superclass constraint from Variate type class
Remove Unbox superclass constraint from Variate type class

It's not used anywhere and in fact harmful since data types
which aren't instances of Unbox couldn't be instances of
Variate which doesn't really make sense.

On GHC6.12 removal of Unbox contraint decreased performance
significantly ~10x

With GHC7.0 performance in fact slightly improves ~0-25% on
64bit and <10% for 32bit (It's swamped by slow 64 arithmetics)

bos/mwc-random,Shimuuar/mwc-random,bos/mwc-random,Shimuuar/mwc-random