simulating dynamic dispatch

[Hal Daume III]

> -- *Main> test $ MkFoo (0::Int)
> -- Just True
> -- *Main> test $ MkFoo (10::Int)
> -- Just False
> -- *Main> test $ MkBar 'a'      
> -- Just True
> -- *Main> test $ MkBar 'b'
> -- Just False

ah, yes.  i was aware that would work.  i forgot to mention the constraint
that i don't want the user to have to use the MkFoo/MkBar
constructors.  if i could use them internally to 'test', that would be
great, but that's what i couldn't get to work :).