Move moment index into circuit message (#7008)

* Move moment index into circuit message

- This addresses design review comments that the moment_index should
be outside the Moment message to be consistent and avoid a message
construction.
- This PR also simplifies the code to deserialize all the constants
first, which removes the need for lookup tables and logic later.
- In addition, this puts the deserialization of a moment into its
own function.

* types

* typo]]]]

* Fix build protos to whatever version thing I was missing.

* Address review comments.

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

@@ -70,19 +70,20 @@ message Circuit {
   // The moments of the circuit, with the first element corresponding to the
   // first set of operations to apply, etc.
   repeated Moment moments = 2;
+
+
+  // The index of the moment in the top-level constant table.
+  // In order to preserve ordering, either this field should be populated
+  // or the moments field, but not both.
+  // This field is used to reduce size of circuits that contain many
+  // repeated moments.
+  repeated int32 moment_indices = 3;
 }
 
 // A moment is a collection of operations and circuit operations that operate
 // 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;
@@ -97,6 +98,9 @@ message Moment {
   // table should be preferred for circuits with repeated operations
   // for improved serialization size.
   repeated int32 operation_indices = 4;
+
+  // Deprecated field id, do not use.
+  reserved 3;
 }
 
 // The quantum circuit, specified as a series of operations at specific