(isearch-unread-key-sequence): New function, extracted from

(isearch-unread-key-sequence): New function, extracted from
isearch-other-meta-char.
(top level): (put 'foo 'isearch-scroll) on all Emacs's
"scrollable" standard functions.
(isearch-allow-scroll): New customizable variable.
(isearch-string-out-of-window, isearch-back-into-window)
(isearch-reread-key-sequence-naturally)
(isearch-lookup-scroll-key): New functions.
(isearch-other-meta-char): Doc string and functionality enhanced.
Now accepts a prefix argument.
(isearch-lazy-highlight-window-end): New variable.
(isearch-lazy-highlight-new-loop): Pay attention to the window's
end (thru isearch-lazy-highlight-window-end), not only its start.