Add StatefulGen instance for Gen

[lehins]

This is a preliminary PR to see if this functionality is even desired. In case that maintainers will see this as useful addition I can add some haddock and examples. Also QC was just the very first thing that came to mind, it can be renamed to something totally different.

This small change allows to use all of the stateful interface from new version of >= random-1.2.0 directly from within the Gen monad. For example:

• Generate anything with instance Uniform

  λ> import System.Random.Stateful (uniformM, uniformRM, uniformShortByteString, uniformListM)
  λ> sample (uniformM QC :: Gen Int)
  -6952550179692266586
  -118868272850968943
  ...

• Generate anything with instance UniformRange

  λ> import System.Random.Stateful (uniformRM)
  λ> sample (uniformRM (0, 10) QC :: Gen Double)
  8.829122155155709
  7.057056838467508
  9.1976303653651
  ...

• Use other custom functions that generate random data:

  λ> import System.Random.Stateful (uniformListM)
  λ> sample (uniformListM 5 QC :: Gen [Int])
  [-332073426443689,2129047816008679820,-7574432746071061604,-7820282781062474525,2921130720166821963]
  [-449122374714186863,-6428736678288918214,5352387993016661666,2292716122803412839,-7086338435263495393]
  [-1122107547762043612,9027060838827498759,6642931938644004524,5993303351323562059,-274895484945480833]
  ....

• Efficient generation of binary blobs:

  λ> import System.Random.Stateful (uniformShortByteString)
  λ> sample (uniformShortByteString 10 QC :: Gen ShortByteString)
  "\175~\v\174\141[/\\U{"
  "\STXV\NAK\FS\SOH\165^\141\163\&4"
  "\245\182\183\158\152.E\232\162:"
  ....
  λ> sample (uniformByteStringM 10 QC :: Gen ByteString)
  "+\185\145\STX\202\239\ETB0\190W"
  "\STX\192})\241Cr\192\194\EOT"
  ...

Of course, instead of exporting QC we could just create specialized functions, i.e. uniform :: Uniform a => Gen a etc. but that is less useful IMHO, because it would prevent usage of other general functions that people could write for StatefulGen g m

[MaximilianAlgehed]

What's the rationale for having StatefulGen QC Gen and not StatefulGen QCGen Gen?

[MaximilianAlgehed]

What's the rationale for having StatefulGen QC Gen and not StatefulGen QCGen Gen?

Never mind, looking closer at the StatefulGen interface I get it now.