changed the implementation of SACargs completely to improve the FFI

SACargs can now be used when calling C functions from SaC as well. To facilitate this, I also added global variable of type SACtypes *. These improve vastly over the old method of encoding UDTs.

This (hopefully) will also help when integrating structural types properly.

Some more polishing can follow, but the basic principles are already there and we still have enough backward compatibility that the old Stdlib still compiles. Once that has been re-written, we can abandon all support for #pragma refcounting in favour of the use of the new #pragma sacarg.