Adjust for new additions in upcoming random-1.3

[lehins]

Hey @Shimuuar I am working on some changes in the random-1.3 stateful interface in https://github.com/haskell/random/pull/153 Could you take a look and see if you find it sensible.

Addition of modifyGen is more useful for stateful generators that are regular wrappers around RandomGen rather than for the ones like mwc implements. It also allows for a mutable Gen to be overwritten with the Seed using overwriteGen

Depends on #88