Fix cont associativity test: = function only had one argument passed

* wraping both forms in the equals meant the test failed, fixed by
  wrapping the bind calls in run-cont and changing `inc-cont-fn` to
  `(inc-cont-fn x)` in the mlet
Merge pull request #25 from minimal/fix-cont-monad-test

Fix cont associativity test: = function only had one argument passed