Streamline proto serialization of circuits with duplicate operations (#…

…6991)

* Streamline proto serialization of circuits with duplicate operations

- Some circuits, such as randomized benchmarking circuits, have many
repeated operations.
- Streamline the proto serialization of these circuits so that these
operations 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.

* fix type

* Addressed some of the comments.

* coverage

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

@@ -45,6 +45,9 @@ message Constant {
 
     // Moments used multiple times in a circuit
     Moment moment_value = 4;
+
+    // Operations used multiple times in a circuit
+    Operation operation_value = 5;
   }
 }
 
@@ -87,6 +90,13 @@ message Moment {
   // All of the circuit operations in the moment. Each operation and circuit
   // operation must act on different qubits.
   repeated CircuitOperation circuit_operations = 2;
+
+  // All of the operations in the moment that are stored in the constants
+  // table.  Each operation should be stored in either `operations`
+  // or `operation_indices`.  Putting operations into the symbol
+  // table should be preferred for circuits with repeated operations
+  // for improved serialization size.
+  repeated int32 operation_indices = 4;
 }
 
 // The quantum circuit, specified as a series of operations at specific