Most of this is trivial -- e.g. getting rid of dummy definitions of
coarbitrary, which is a separate typeclass in QC2, and changing the
parameter specs to use the new Args type.
The chunk of code that runs a QC test as an HUnit test is now quite a
bit simpler because QC's top-level interface returns a result. However,
this means that some gymnastics are required to get at the right
definition of Result in different places -- QC2 has two different types
called Result, and we use both for different jobs!
