Avoid QuickCheck for properties without free variables

[bjornbm]

Properties that have no free variables and have type Bool can be checked
to be True without using QuickCheck. Properties with free variables need
QuickCheck. Some properties may not have free variables but if the type
is not Bool we use QuickCheck on the assumption that they are of some
other Test.QuickCheck.Testable instance.

The purpose of this pull request is to be able to benefit from the prop> syntax for simple properties without pulling in QuickCheck as a dependency.

An example in the wild:

--   >>> isRight (refine @(Not NonEmpty) @[Int] [])
--   True
--
--   >>> isLeft (refine @(Not NonEmpty) @[Int] [1,2])
--   True

Can be clearer (IMO) and more concisely written as:

prop> isRight (refine @(Not NonEmpty) @[Int] [])
prop> isLeft (refine @(Not NonEmpty) @[Int] [1,2])prop> 

(I will note that even when QuickCheck is not used prop> will be slower than >>> due to calling the repl twice.)

[bjornbm]

Sorry, that failing test case got lost amongst noise related to cabal-v2 and GHC-8.6.5 on my local machine.

[quasicomputational]

Hmm. I can appreciate this as an optimisation, but the UX worries me a shade. At the moment, the rule is simple: if you use prop>, you need a QuickCheck dependency. With this patch, the rule gets more complicated. Is the win in test speed and not needing the QC dependencies sometimes worth the decrease in uniformity?

[sol]

If somebody is willing to rebase this changes and is OK with some bike shedding during review, then I'm very much willing to accepting this patch.