Deprecate RandomGenM in favor of a more powerful FrozenGen

[lehins]

• Addition of modifyGen totally removes the need for RandomGenM, because every frozen generator that is a netwtype wrapper around RandomGen also derives an instance for RandomGen
• Add splitMutableGen and splitGen
• Add ThawedGen and move thawGen into that type class to combat current unlawful FrozenGen instance for StateGen

[Shimuuar]

That's great! It provides direct correspondence between RandomGen based API and StatefulGen one. Just a few thoughtsfrom me:

1. modifyM clashes with one from transformers state monad. modifyGen?
2. This addition mean that one could implement thawGenInplace :: MutableGen f m -> f -> m () which replaces existing mutable generator.

[lehins]

1. modifyM clashes with one from transformers state monad. modifyGen?

modifyGen sounds good to me.

2. This addition mean that one could implement thawGenInplace :: MutableGen f m -> f -> m () which replaces existing mutable generator.

Yes, that seems like a good feature to me.

[lehins]

This addition mean that one could implement thawGenInplace :: MutableGen f m -> f -> m () which replaces existing mutable generator.

@Shimuuar I've added overwriteGen :: MutableGen f m -> f -> m () to make this feature explicit and allow for more efficient implementation, like in this PR: haskell/mwc-random#89 with copy