Unroll Loop Core; Reduce Frequency of Repcode Check & Step Calc (+>1% Speed)

Unroll Loop Core; Reduce Frequency of Repcode Check & Step Calc (+>1% Speed)

Unrolling the loop to handle 2 positions in each iteration allows us to reduce
the frequency of some operations that don't need to happen at every position.
One such operation is the step calculation, which is a very rough heuristic
anyways. It's fine if we do this a position later. The other operation is the
repcode check. But since the repcode check already tries expanding back one
position, we're really not missing much of importance by only trying it every
other position.

This commit also slightly reorders some operations.