Fix ABI compatibility with C

[fjolnir]

I just discovered that I'm not properly handling byval passing of structs of floats (and possibly) small ints.
structs like struct { float a,b, c; } need to be expanded to <vector 2x float>, float in order to be passed to a C function.
There are similar cases for other types.

DragonEgg&Clang might be helpful resources

[fjolnir]

For now this is avoided by going through libffi (which is much much slower of course)