Monadic iteration complete.

Note::
so as it happens running this parser requires using runEval' to throw
the computation into the appropriate Monadic context ( this case being
Eval')
runEval' $ eval'' Map.empty exampleExpress

I've removed the concrete class definition in eval'' and replaced it
with a class constraint instead, which means we we can use eval'' in ANY
monadic context.  Seeing as it doesn't leverage any of the fancier
monadic instructions, we should be fine.

And since ghc runs the app in the IO Monad as a matter of course,
nothing amazing here =O