MAP-INTO optimization opportunity

[Alastair Bridgewater]

From SYS:SRC;CODE;SEQ.LISP, starting at around line 1063:
;;; KLUDGE: MAP has been rewritten substantially since the fork from
;;; CMU CL in order to give reasonable performance, but this
;;; implementation of MAP-INTO still has the same problems as the old
;;; MAP code. Ideally, MAP-INTO should be rewritten to be efficient in
;;; the same way that the corresponding cases of MAP have been
;;; rewritten. Instead of doing it now, though, it's easier to wait
;;; until we have DYNAMIC-EXTENT, at which time it should become
;;; extremely easy to define a reasonably efficient MAP-INTO in terms
;;; of (MAP NIL ..). -- WHN 20000920

We've had DYNAMIC-EXTENT for a while now, perhaps this should be looked into again?