perf: syntax highlighting searches the saved state list from the start

[h-east]

Problem:  Looking up the saved syntax state for a line scans the state
          list from the start every time, which is slow when highlighting
          a large file in increasing line order.
Solution: Remember the last found entry and start the search there when it
          is at or before the wanted line; the list is sorted, so the
          result cannot be earlier.  The highlighting is unchanged.