[WIP] Allow OBSERVABLE_INCLUDE to target Pauli terms

[Strilanc]

TODO: add crumble support

TODO: add diagram support

There are two use cases driving this change:

1. Magic state injection needing requiring partially deterministic observables
2. Simulating all observables of a code without needing to add noiseless ancilla qubits to the circuit

This change allows observables to be split into pieces (e.g. obs 1 for the first half of the circuit and obs 2 for the second half, with the "true" observable their xor). The flip of each individual piece can even be recovered when using flip simulation, if stabilizer randomization is disabled.

This change allows finer control over the logical labels that appear in the detector error model. For example:

import stim
assert stim.Circuit("""
    OBSERVABLE_INCLUDE(0) X0
    OBSERVABLE_INCLUDE(1) Y0
    OBSERVABLE_INCLUDE(2) Z0
    X_ERROR(0.125) 0
    Y_ERROR(0.25) 0
    Z_ERROR(0.375) 0
    OBSERVABLE_INCLUDE(0) X0
    OBSERVABLE_INCLUDE(1) Y0
    OBSERVABLE_INCLUDE(2) Z0
""").detector_error_model() == stim.DetectorErrorModel("""
    error(0.375) L0 L1
    error(0.25) L0 L2
    error(0.125) L1 L2
""")