Add tags to detector error models

[Strilanc]

Adds the ability to tag instructions in detector error models (like error[crosstalk](0.1) D1 D2). Tags have no semantic effect, other than to be propagated. Outside systems can read the tags for custom behavior, or to help with debugging.

stim.Circuit.detector_error_model also now flows tags from the circuit into the dem. Tags on noise instructions end up as tags on error instructions, tags on detectors are kept, and so forth.

For example:

print(stim.Circuit("""
    R 0
    X_ERROR[hello-world](0.25) 0
    M 0
    DETECTOR[alarm] rec[-1]
""").detector_error_model())

prints:

error[hello-world](0.25) D0
detector[alarm] D0

Fixes #872