Missing instance SplitGen g => SplitGen (StateGen g)

[treblacy]

Version 1.3.0 is missing an instance SplitGen g => SplitGen (StateGen g).

It could be easily fixed in the source code by visiting newtype StateGen g = ... deriving (..., RandomGen, ... and adding SplitGen, much like how it's done in IOGen, STGen, etc.

Thanks.

[lehins]

@treblacy you are absolutely right. This was missed when SplitGen was added.
Would you like to submit a PR that adds this as a derived instance to StateGen?
If not, it's ok, I'll get it done and make a release over the next weekend

[treblacy]

OK done! #183