Add concise serialization tools.

[95-martin-orion]

Part of #3235. Also contributes to #3438.

This PR adds a SerializableByKey interface which any Cirq object can implement to enable more concise JSON serialization of repeated objects. Enabling this behavior for various types is left to future PRs.

For comparison, consider MatrixGate:

a, b, c = cirq.LineQubit.range(3)
op = cirq.MatrixGate(some_big_matrix)
circuit = cirq.Circuit(op.on(a), op.on(b), op.on(c))
cirq.to_json(circuit)

Without concise serialization for MatrixGate:

Details

{
  "cirq_type": "Circuit",
  "moments": [
    {
      "cirq_type": "GateOperation",
      "gate": {
        "cirq_type": "MatrixGate",
        "matrix": some_big_matrix
      }
      "qubits": a
    }
    {
      "cirq_type": "GateOperation",
      "gate": {
        "cirq_type": "MatrixGate",
        "matrix": some_big_matrix
      }
      "qubits": b
    }
    {
      "cirq_type": "GateOperation",
      "gate": {
        "cirq_type": "MatrixGate",
        "matrix": some_big_matrix
      }
      "qubits": c
    }
  ]
}

With concise serialization for MatrixGate:

Details

# If MatrixGate implements SerializableByKey, this becomes:
{
  "cirq_type": "_ContextualSerialization",
  "context_list": [
    {
      "cirq_type": "_SerializedContext",
      "key": op._serialization_key_()
      "value": {
        "cirq_type": "MatrixGate",
        "matrix": some_big_matrix
      },
    },
    {
      "cirq_type": "Circuit",
      "moments": [
        {
          "cirq_type": "GateOperation",
          "gate": {
            "cirq_type": "_SerializedKey",
            "key": op._serialization_key_()
          },
          "qubits": a
        },
        {
          "cirq_type": "GateOperation",
          "gate": {
            "cirq_type": "_SerializedKey",
            "key": op._serialization_key_()
          },
          "qubits": b
        },
        {
          "cirq_type": "GateOperation",
          "gate": {
            "cirq_type": "_SerializedKey",
            "key": op._serialization_key_()
          },
          "qubits": c
        },
      ]
    }
  ]
}

Note that some_big_matrix appears three times in the current serialization, but only once with concise serialization.

[balopat]

Thanks for this! Added some comments!

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