Adds benchmarks/reverse4x to manually examine the effects of unrolling.

Running on magellanic.
------ Config refcnt-opt ------
reverse              0:05.14
reverse4x            0:01.27
------ Config refcnt-lt-opt ------
reverse              0:05.37
reverse4x            0:01.33
------ Config gc-opt ------
reverse              0:01.92
reverse4x            0:00.49
------ Config gc-lt-opt ------
reverse              0:03.03
reverse4x            0:00.75
------ Config ghc ------
reverse              0:08.71
reverse4x            0:00.70
------ Config mcc ------
reverse              0:04.48
reverse4x            (failed)