Streamline proto serialization of circuits with duplicate moments. (#6941)

- Many circuits, in practice, have moments that are repeated in the
circuit. For instance, XEB are long circuits with duplicate layers of 2
qubits gates. Long QEC experiments also have duplicate moments.
- Streamline the proto serialization of these circuits so that these
moments are stored only once (in the constants table).

Note that this PR only adds deserialization by default and hides
serialization behind a feature flag. Once this is deployed everywhere,
then we can enable serialization.

Author: dstrain115

cirq-google/cirq_google/api/v2/program.proto

@@ -42,6 +42,9 @@ message Constant {
 
     // Qubits used within the circuit (only populated in v2.5+)
     Qubit qubit = 3;
+
+    // Moments used multiple times in a circuit
+    Moment moment_value = 4;
   }
 }
 
@@ -70,6 +73,13 @@ message Circuit {
 // on a disjoint set of qubits. Conceptually, a moment represents operations
 // that all occur in the same finite period of time.
 message Moment {
+  // The index of the moment in the top-level constant table.
+  // If this field is populated, the other fields should be ignored
+  // in favor using the constant table lookup.
+  // This field is used to reduce size of circuits that contain many
+  // repeated moments.
+  optional int32 moment_constant_index = 3;
+
   // All of the gate operations in the moment. Each operation and circuit
   // operation must act on different qubits.
   repeated Operation operations = 1;