Possibly keep an array sorted instead of sorting-all-at-once in gather_anti_quotes

From a FIXME in `syntax::ext::qquote`: "Maybe this is an overkill (merge_sort), it might be better to just keep the gather array in sorted order ..."

I'm suspecting that the arrays in question might not ever be large enough for it to make a difference in performance anyway. But I guess someone might want to do a comparison.
