This performs better with "real" code (the Par monad) but surprisingly does far
worse with the included tests! The next thing to do is implement the orthogonal
sleep sets algorithm to cut down on available choices even further and
hopefully correct this issue.
See also: "Bounded Partial-Order Reduction" [Coons, Musuvathi, McKinley 2013]
This allows results to be naturally reported as lazy trees, rather
than as lists representing a tree traversal. This in turn means
that the actual bound can be moved outwards to the testing code, and
not used at all in the runner. Trees let us do nice things with
shrinking and short-circuiting, if we make the (fairly reasonable)
assumption that the children of a buggy result will exhibit the same
bug.
Storing results as trees does complicate the predicate helper
functions somewhat, but I think the clarity gained in the actual
SCT code is well worth it.
