Modernize SMX headers.

I took the newer headers from the SP repo and threw them in sourcepawn/include for now. I didn't feel like changing the compiler (which needs a transition to c++, really) so it has an old copy of sp_file_headers.h locally. I just didn't want to expose it everywhere.

A few SMX structs are used directly in the SP API. I inlined them into the old API files.