Provide a C binding for PPMP

[kartben]

It would be useful to have a C library that people can use to serialize/unserialize PPMP message, and therefore use the protocol on microcontrollers, etc.

[fpatz]

If we can restrict the C implementation to one direction (sending), we would not need a JSON parser, and it would be straightforward to do an implementation in C.

[ameinhardt]

@azoitl: would you provide a C binding?