Many compile errors due to IMP caching on Apple

[iamleeg]

Modern clang compilers targeting the Apple runtime (sorry, I don't know since which version exactly, except that Catalina 10.15 behaves like this) expect IMPs to be called in a type-safe fashion. The type is defined as void (*IMP)(void) and to use one, you have to cast it to e.g. id (*valueForKeyIMP)(id, SEL, id) or whatever type you need.

The uses of IMP caching throughout GDL2, both through static inline functions and compiler macros, thus cannot be compiled on macOS. I'd be happy to submit a PR, but having realised how big a yak to shave this is, wanted to discuss three possible implementations and see which is preferred:

1. remove all the IMP caching. If you know that there's a current use case for GDL2 where going through the ObjC runtime is too slow then of course this is a non-starter.
2. Use the runtime API for fast method invocation on the Apple runtime. I don't know what the performance implications of this vs. IMP caching are.
3. insert casts in every header where GDL2 currently defines an IMP-cache-using function or macro.