Fixed a performance problem with unif_machines and small_delta_step.

Fixed a performance problem with unif_machines and small_delta_step.
Previously, small_delta_step would occasionally return two machines in
normal form, with a false "are_normal" flag: this in turn caused
unif_machines to perform more steps than necessary. Now the flag returned
by small_delta_step is true iff it could not perform any reduction step;
therefore unif_machines won't try a recursive call on the new machines
(which are equal to the old ones).