catch/throw performance loss

["Scott E. Fahlman" <Fahlman@CMU-CS-C.ARPA>]

All that random users need to remember is that Catch tags are supposed
to be symbols.  We generalized it a bit beyond that, because we could do
this at *NO* extra implementation cost and it made some odd cases a bit
cleaner to use some fresh-consed object other than a symbol. But rather
than generalize to a case that IS going to cost more, I'd retreat to
allowing symbols only.

-- Scott