Upgrade to Scalaz 7.10 and all that entains

Started upgrading Mundane and hit this. There are two changes I made that I think are _wrong_ and would appreciate feedback on - I'll leave comments on the specific lines.

/cc @etorreborre @markhibberd 
`return_` requires an applicative for `RngOp` (which has a `Comonad` instance). Do you think it is possible to write one that makes sense? If you could find some time to fix this PR with some compiling code that'd be very appreciated :-).