Implementation of TGenM for STM

[lehins]

I don't know why I haven't thought of this before, but here is a mutable generator that works in STM: TGenM that is based on a TVar

[lehins]

Anyone feels like reviewing this PR? If not, I'll merge it over the weekend and make it part of the upcoming release #110

[Shimuuar]

Looks good. It adds dependency on STM but since it's de facto part of stanadrd library it's not a problem.

I thought it could cause problems for STM if its test suite depends on random and thus becomes nonbuildable. But it turns out stm does not have one, at least in cabal file

[lehins]

It adds dependency on STM but since it's de facto part of stanadrd library it's not a problem.

Actually only the test suite depends on stm. The implementation itself imports TVar directly from base: import GHC.Conc.Sync:
https://github.com/haskell/random/pull/95/files#diff-2c181ac3f73b5e3eadd5b0019d00d2a54ecbcc244a8d583a8d2aeb1cad97eda1R110