Control/Monad/CryptoRandom.hs: import Data.Tagged to fix the build with tagged 0.7

The Data.Proxy module used to export the 'proxy' function, but in tagged
version 0.7 this function is available only after importing Data.Tagged.
Apparently, this change in tagged is necessary to prepare for the
upcoming GHC 7.7, which is going to ship Data.Proxy as part of base.
Merge pull request #5 from peti/master

Control/Monad/CryptoRandom.hs: import Data.Tagged to fix the build with tagged 0.7