defclass: make sure that defclass option "NIL" errors
This is as for CLHS section 7.1.2. Error was caused by the fact that unknown-key was a flag, so if the initarg was NIL, we were assigning it value NIL, what is also a boolean false. Right now we collect all invalid initargs in a list, so in case of NIL we'll get (NIL) what is a generalized boolean true value. Closes #474. Also add cosmetic fixes with declarations.
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