Some nice Monad examples from "A Fistful of Monads"

Lots of nice monad stuff here.

* `threeCoinsRevisited.hs` introduces `state random` -- an improvement
on `threeCoins.hs` in that we need not pass in the new generator in the
random function -- the State monad keeps the latest generator for us.

* `addLogging.hs` has been updated to feature a Criterion benchmark.
Output is in `gcd.html`.

* `safeRPN.hs` shows an interactive RPN calculator that withstands bogus
user input (unlike `rpn.hs`).