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

IEC-61131 PLC programming
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Contents:

.. toctree::
   :maxdepth: 2

   create
   configuration
   edit
   connect
   run
   stdlib/index