Proto serialization v25 (#4333)

This PR creates a new class called CircuitSerializer that can serialize Circuits into protocol buffers more efficiently than existing mechanisms.

This class is similar to SerializableGateSet and op_(de)serializer, in that it serializes and deserializes cirq.Circuits into Program protocol buffers, but it is about 60% more space efficient and has significant time savings as well.

This serializer works by changing the Gate field and Args dict to a specific message for each allowed gate type.  It changes Qubits from a repeated Qubit message, to an index in the constant message.

This PR also adds a FloatArg message which is specific to Arguments where it is known the value will be a float (or a sympy expression).

Author: dstrain115

cirq-google/cirq_google/__init__.py

@@ -111,6 +111,10 @@
     optimized_for_xmon,
 )
 
+from cirq_google.circuit_serializer import (
+    CircuitSerializer,
+)
+
 from cirq_google.op_deserializer import (
     CircuitOpDeserializer,
     DeserializingArg,