MQTT: add JSON payload corresponding to IEC Structured types

- uses https://github.com/cesanta/frozen
- frozen.c is added as a resulting c file in generat_C (fix toolchain_gcc.py as it was breaking dependency checking)
- structured types are converted to C #define, in turn generating json_scanf and json_printf statements

# opcua/__init__.py



from .client import OPCUAClient

# class RootClass(object):
#     XSD = """<?xml version="1.0" encoding="ISO-8859-1" ?>
#     <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
#     </xsd:schema>
#     """
#     CTNChildrenTypes = [("OPCUAClient", OPCUAClient, "OPCUA Client")]
#