Enable async procedures to use function pointer + user data pointer instead of std::function

[eliemichel]

Currently, async functions, i.e., functions that take as last parameters a callback and user data pointer, are translated in the cpp wrapper into a std::function, but one may sometimes want to be able to access the original lower level function pointer + user data pointer signature. We should then generate an overload of these async fonctions that exposes such an API.

[eliemichel]

Side note: now that the callback mechanism take 2 userdata pointers, we can also improve the way we pass closures (capturing lambdas) through the C API.