Function: eieio-add-new-slot

Add into NEWC attribute A.
If A already exists in NEWC, then do nothing. If it doesn't exist,
then also add in D (default), DOC, TYPE, CUST, LABEL, CUSTG, PRINT, PROT, and INIT arg.
Argument ALLOC specifies if the slot is allocated per instance, or per class.
If optional DEFAULTOVERRIDE is non-nil, then if A exists in NEWC,
we must override its value for a default.
Optional argument SKIPNIL indicates if type checking should be skipped
if default value is nil.