BASE-CHAR-P, called by TWO-ARG-CHAR-EQUAL, isn't properly implemented
on non-sb-unicode, the transform by which it gets transformed into
(typep x 'base-char) is disabled, causing an infinite loop.
Since testing for base-char-p is usually redundant on #-sb-unicode,
don't define it there at all. This will catch inadvertent uses.
In the few places where it's currently used, it's can be safely