Add arrows to paired string delimiters

Unicode has lots of arrows of various shapes, sizes, and directions.
None of them were of consequence to the Bidirectional algorithm, so none
were specified as being mirrored pairs.  This commit uses the
generalizations already in place from previous commits to examine arrow
symbols and choose which are mirrored pairs.

As previously, it rejects arrows with contrary directionality, and ones
without horizontal directionality.