Allow more specific foreign types

[Moritz Heidkamp]

Currently, typedefs cannot be used directly but have to be replaced by an approximated equivalent FFI type, e.g. typedef unsigned int foo would have to to be referred to as unsigned-int yielding some compiler warnings. Even worse: If the type is defined differently for different platforms, say, it might not work at all. My suggestion is to allow a similar construct like (struct "foo") to say (unsigned-int "foo"). With this the compiler would know what foreign type to use but insert the correct typedef name in the generated code.