PyObjC support for vector/SIMD types

Apple uses vector/SIMD types, like vector_float3, in a number of APIs. These types are not yet supported by PyObjC. Support will be added in a future versions.

The technical reason for the lack of support is twofold:

  1. The low-level library used to construct C function calls (libffi) does not support these types.
  2. The Objective-C compiler does not include information about arguments/return values of these types in the runtime metadata.

The latter point is primarily annoying and can easily be circumvented using PyObjC’s metadata system. The first point is more serious and requires significant work to be able to support these types.