Adds the wheel_sieve2 benchmark.  During debugging, I noticed that there are
long chains of FWD nodes that are not compressed.

Running on yellow-eyed.
------ Config refcnt-opt ------
reverse              0:01.11
tak                  0:01.83
rfib                 0:00.30
exp3_8               0:00.20
tree_insert          0:00.94
tree_insert_32       0:00.93
primes               0:02.27
queens               0:00.45
wheel_sieve1         0:00.27
wheel_sieve2         0:03.74
------ Config gc-opt ------
reverse              0:00.66
tak                  0:01.86
rfib                 0:00.28
exp3_8               0:00.10
tree_insert          0:01.51
tree_insert_32       0:01.48
primes               0:02.45
queens               0:00.32
wheel_sieve1         0:00.23
wheel_sieve2         0:01.66