ObjFW runtime: Add objc_msg_lookup(_super)_{fp,st}ret

[Jonathan Schleifer]

I just noticed that in this patch, I also handle fpret differently, even though that should not be necessary for forwarding (or is there an architecture where it is?). The idea was originally that this would make sending to nil more robust, however, what an fpret is is highly platform dependent. Therefore I'm not sure if also having objc_msg_send_fpret is a good idea. Any opinions on that? (I think the compiler should handle nil messaging for everything that does not return an integer type - this seems to be the only way to always do it right).

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Jonathan