Use xeus's routines for creating the replies

[arjxn-py]

Fix #522

[arjxn-py]

cc: @JohanMabille @martinRenou

Is this how we want to use the xeus's routines?

[JohanMabille]

These routines are meant to be used in the xinterpreter.cpp and xinterpreter-raw.cpp file. Except for create_successful_reply and create_error_reply that must be used in execute_request_impl, the other method names matches the request handler names.

We don't have the equivalent of these methods for the debugger (but it might be worth creating them in the future).

[arjxn-py]

@martinRenou @JohanMabille This should be ready for your first review, please let me know if i am missing anything or overdid anything.

[martinRenou]

Thanks!