[swift2objc] Filtering

[liamappelbe]

Like ffigen, swift2objc should have config options that let the user choose which APIs they want to generate wrappers for. Real Apple APIs have so many classes and methods that it's not practical to generate everything.

The Config should have callbacks that the user can set which return whether a particular class/struct/method/property should be generated. By default we should exclude everything.

[liamappelbe]

It'd be nice if we can do #1358 first and then implement filtering as a transformer

My experience from ffigen is that using transformers for this is more trouble than it's worth. Better to just parse everything into a flat list of all bindings, then do a pass over that list applying the filter.