Generate files in parallel

[posener]

Hi Thanks for this great library. I'm generating ffjson files as part of my build. Currently the API support generating only one file at a time, and it takes much time. If I try to generate in parallel (using make -j for example) the generation fails FFJSONExpose redeclared in this block. The current solution is to move all the structs into one file, but it would be nice to be able to handle it in the ffjson API. Cheers.