Fix first half of lp#1354606 for splicing in quasiquoted vectors.
Fix undefined behavior case of lp#1354606
Fix lp#1354623 - macroexpansion of QUASIQUOTE broken by rev.88411384
Transform (STRING sym) to (SYMBOL-NAME sym)
Fix WITH-PACKAGE-ITERATOR for empty list of packages
":IMPORTANT NIL" in DEFTRANSFORM now means really unimportant.

Users aren't likely to be able to make code more efficient when
shown "The first argument is a T, not a (NOT NULL)" so don't.
(It only worries them)