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 
Hi Tony can you please follow up on this with the rest of the Scalaz team? RNG is one of our dependencies and it's holding us back from being able to uniformly upgrade to Scalaz 7.1.0. Thanks.