* allout.el (allout-hide-by-annotation, allout-flag-region): Reduce

* allout.el (allout-hide-by-annotation, allout-flag-region): Reduce
possibility of overlay leakage by making them volatile.

* allout-widgets.el (allout-widgets-tally): Define as nil so the hash is
not shared between buffers.  Mode initialization is responsible for giving
it a useful starting value.
(allout-item-span): Reduce possibility of overlay leakage by making them
volatile.
(allout-widgets-count-buttons-in-region): Add diagnostic function for
tracking down overlay leaks.