Do highlighting of alternative matches all at once,

Do highlighting of alternative matches all at once,
but only do it in the text that's visible in the selected window.

(isearch-lazy-highlight-cleanup): Arg now says
to remove all the current alternative-match highlighting.
If nil, remove only what's outside the current window.
(isearch-lazy-highlight-remove-overlays): Take optional
region within which NOT to remove them.
(isearch-lazy-highlight-new-loop): Greatly simplified.
(isearch-lazy-highlight-update): Find all the other occurrences
visible in the window, in just one call.

(isearch-lazy-highlight-start): Now holds start of region to scan.
(isearch-lazy-highlight-end): Now holds end of region to scan.
(isearch-lazy-highlight-wrapped): Variable deleted.
(isearch-lazy-highlight-search): Function deleted.