Generate FFI calls procedurally

[iduartgomez]

Since the library was initally developed a lot of new goodies have made it into the language and ecosystem, amongst them procedural macros, with this is possible to achieve a couple things:

• Move from 'prefix based' annotation for func calls to attribute annotated func calls when discovering interfacing.
• At compile time parse any function that requires interfacing without explicitly declaring the FFI wrapper and generate it on the flight (I believe PyO3 does something similar?).

Adding infrastructure for the latest would open more possibilities to handle more complex types (like nested structs) probably too, and the basics are there is librustypy already.