Add circuit method to get the "detecting region" of a detector or observable

[Strilanc]

This is basically what the detector error model extraction does, except I want to record where the detector went instead of only which errors it touched.

This will probably require adding classes to represent the result. In particular I want to be able to draw diagrams of the region to aid with debugging broken circuits.

What makes this a bit tricky is that, ideally, it would be able to quickly find the relevant measurements. In particular, it should probably be able to skip over big but irrelevant or boring loops.