bug#41354: equal? has no sensible code path for symbols

In Scheme, symbols can be compared using eq? for equality.  However,
since they have garbage-collected content attached, they do not meet the
predicate SCM_IMP in the short-circuit evaluation at the start of equal?
This means that unequal symbols compared using equal? fall through a
whole bunch of tests and end up in a general structural comparison
comparing their underlying string names.

This completely sabotages the semantics symbols are intended for.
Behavior for eqv? is similar but the fall-through at least is not as
expensive as it is for equal? .

-- 
David Kastrup