NewCalculatorServiceClient creates an RPC client that conforms to the CalculatorService interface, but delegates work to remote instances.

NewCalculatorServiceGateway accepts your "real" CalculatorService instance (the thing that really does the work), and exposes it to other services/clients over RPC.

CalculatorServiceClient manages all interaction w/ a remote CalculatorService instance by letting you invoke functions on this instance as if you were doing it locally (hence..

CalculatorServiceProxy fully implements the CalculatorService interface, but delegates all operations to a "real" instance of the service.