FFI targetting C

[Mesabloo]

This one will be difficult, but is being discussed in #4 (comment).
We have to fix a complete ABI in order to have predictable behaviors when linking with/using C code (maybe even restrict naming conventions when compiling foreign exported functions?).

This should be the most difficult part of the first version of Zilch (we may also have to change some parts of N⋆ for this to work).