Add the ability to tag circuit instructions

[Strilanc]

• For example, TICK[100ns] instead of TICK or I[DYNAMICDECOUPLE] 2 3 5 instead of I 2 3 5
• Stim ignores tags, except for propagating them through circuit transformations
• Users can use tags for debugging or to communicate information from tools generating stim circuits to tools consuming them

import stim
circuit = stim.Circuit("""
    R[mlr] 0 1
    TICK[200ns]

    H 0
    TICK[10ns]

    REPEAT[unroll] 3 {
        CZ[adiabatic] 0 1
        TICK[20ns]
    }

    H 0
    TICK[20ns]

    I[DYNAMIC_DECOUPLE_NEW_LATEST_VERSION_5_FINAL_ACTUALLY_FINAL_YXY] 2 3 4 5
    M[include_two_state] 0 1
    DETECTOR[subgraph_2] rec[-1] rec[-2]
""")
assert circuit[0].tag == "mlr"
assert circuit[1].tag == "200ns"
assert circuit[2].tag == ""

Fixes #843