Upgrade float to double in cirq protos

Hi, I am suffering from the precision issue due to google protobuf.

In fact, google protobuf float is float32, and double is float64. However, python uses float only for float64, and this makes discrepancy when I saved into protobuf and reload the circuit from the proto. As it happens to others (https://github.com/quantumlib/Cirq/issues/2728, for example) I suggest that upgrading the float to double in cirq protos.

For example, https://github.com/quantumlib/Cirq/blob/master/cirq/google/api/v2/program.proto#L146

float float_value = 1; # -> double float_value = 1;

OR at least, I hope to have double_value as one of the variables,

https://github.com/quantumlib/Cirq/blob/master/cirq/google/api/v2/program.proto#L145-L149

oneof arg_value {
    float float_value = 1;
    RepeatedBoolean bool_values = 2;
    string string_value = 3;
    double double_value = 4; # <- like this.
  }

[dstrain115]

Not urgent, but while you are in the protos, can you check this issue, @verult ? I believe that this has been done and that the resulting (deprecated) fields can be removed, but this could use a bit of verification, since I haven’t touched this in a while.

[dstrain115]

I’ve run into this before when serializing into FSIM_GATESET and then deserializing on the other side, since it wants to check if exponent is close to, say pi/6, and it is only within 1e-6 instead 1e-8 due to the down-scaling to 32 bit precision.