Add double-line diagrams for control keys

[daxfohl]

Adds double-line wires to diagrams between measurement gates and their associated control gates.

• Cbits are represented similarly as qubits with a double-line axis.
• The measurement key name is the label of the cbit (and the key label from M is removed).
• @ is the symbol for writing to the cbit.
  • Currently we only allow write-once, but in the future we may want to allow overwrite, xor, bit-and, sum, etc.
• ^ is the symbol for reading from / executing conditionally upon a cbit.
• X below is hard-coded in the test case, but will be the conditional operation applied.
• Measurement keys that are not used in control operations will not have a line associated, to reduce clutter.

For example a circuit

cirq.Circuit(
    cirq.measure(q0, key='a')
    ControlOp(qubits=[q1], keys=['a'])
)

would have the following representation.

0: ───M───────
      ║
1: ───╫───X───
      ║   ║
a: ═══@═══^═══

See circuit_test for more examples

Parts 6 of https://tinyurl.com/cirq-feedforward.

[daxfohl]

@95-martin-orion This is good to go. It's not an easy PR; if there's anyone that knows more about diagrams then their input may be useful.

There are also a number of variables still named qubits and qubit_map that now contain both qubits and cbits, and should probably now be called xbits or something. I intentionally left those alone so as not to distract from the meat of the PR. I'd recommend dealing with that in a later PR.

Also, duly noted we never did formally specify what these should look like. I provided some examples in circuit_test. Hopefully this is somewhat along the lines of what you were thinking.

[95-martin-orion]

Overall pretty happy with how this turned out. I agree that we should get input from someone familiar with the diagram-drawing tools, though - I'm adding @Strilanc to give this input.

[CirqBot]

Automerge cancelled: No approved review.