QuickCheck 2.7 compatibility

This makes our test compile with out errors with QuickCheck 2.7.
Warnings about the deprecation of printTestCase remain when using 2.7.

This change is backwards-compatible with all older versions of QuickCheck
that we support.

In 2.7, Property is no longer a monad, but remains a `Gen Prop` inside,
so that we only have to use combinations of `property` and `return`
to become compatible.

See
  https://hackage.haskell.org/package/QuickCheck-2.7.6/changelog

Further, in QuickCheck 2.7, Positive/NonZero/NonNegative are no longer
instances of `Integral` (NonNegative could likely still be one, see
https://github.com/nick8325/quickcheck/issues/31).
Consequently we cannot create them using `fromIntegral` any more,
and switch to `fromEnum` instead, which also is backwards-compatible.

Signed-off-by: Niklas Hambuechen <niklash@google.com>