(info-lookup->topic-cache): Use defun, not defsubst.

(info-lookup->topic-cache): Use defun, not defsubst.
(info-lookup->mode-cache, info-lookup->initialized): Likewise.
(info-lookup->completions, info-lookup->refer-modes): Likewise.
(info-lookup->all-modes): Likewise.

(info-lookup-quick-all-modes): New function.
(info-complete): Find the symbol to complete first,
then compute list of completions.

(lisp-mode): Add info-lookup-maybe-add-help for it.