Performance optimazation for rf_find_transitive_hard_core, up 90 times
faster. 10x10 Matrix 7 seconds instead of 158 seconds.
Reason: Instead of generating the whole powerset and afterwards trying
combinations just try combination on the fly and then throw it away.
Data stays in L1/L2 cache instead of being written to main memory (which
is the time taking process) and afterwards reading it.