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.