Simplify s/// and tr/// parsing logic

Simplify s/// and tr/// parsing logic

These two operators were being translated into subst("","") and
tr("","") by the lexer.  Then pmruntime in op.c would take apart the
resulting list op.  Instead of constructing a list op only to take it
apart again, feed the replacement part to pmruntime separately.  We
can achieve this by introducing a new token ('/') that the parser rec-
ognizes as introducing a replacement.

If we had followed this approach to begin with, then bug #123542 would
never have happened.

(Actually, it seems the parser did know about the replacement part to
begin with, but it changed in perl-5.8.0-4047-g131b3ad to fix some
overloading problems.)