catch/throw performance loss

[Glenn S. Burke <GSB @ MIT-MC>]

The fact that catch/throw don't use EQL has always bothered me as a
consistency hole.  I think they should.  I also don't believe that
the tags should be restricted to symbols.  To do this in NIL throw
will have to check to see if it can do the fast EQ lookup -- this is
a fairly trivial check.  I also have some problems with doing hairy
EQL lookup in THROW, but they have to do with maintaining the pointer
to the catch-frame chain during the lookup, because it isn't an explicit
data-type, and EQL can call out to the user.