Add workaround for GHC bug #8072
Add workaround for GHC bug #8072

GHC-7.6 have bug (#8072) which results in calculation of wrongs
number of buckets in function `uniformRange'. Consequently unformR
generates values in wrong range.

Bug only affects 32-bit systems and Int/Word data types. Word32
works just fine. So we set Word32 as unsigned counterpart for Int
and Word on 32-bit systems. It's done only for GHC-7.6 because
other versions are unaffected by the bug and we expect that GHC may
optimise code which uses Word better.

Fixes #25

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