A collection of type-classes generalizing the
read/write/modify operations for stateful variables
provided by things like IORef, TVar, &c.
Note that The interface has changed a bit from the
0.2.* version. "*Ref" functions are now called
"*Reference" and new "*Ref" function exist with
Ref existential type provides a convenient
monad-indexed reference type, and the HasRef class
indicates monads for which there is a default
reference type for every referent.