Reduce reference sample collection time by using loop analysis

[Strilanc]

During reference sampling, in circuits with loops that have a large number of repetitions, the instantaneous stabilizer group can start to recur at the loop boundaries. This can be used to effectively skip simulating the remaining iterations by repeating the reference sample from the previous iterations.

This could noticeably reduce measurement sampling times at large code distances, depending on whether it was the data measurement layers or the stabilizer measurement layers dominating the execution time.

There may be an intermediate regime, where the unsigned tableau recurs but the signs differ. Unsure what's safe to do in that situation.