This change represents Accuracy as a bit pattern rather than
an ordered value; with a new value Undef which is both Below
and Above.

Change-Id: Ibb96294c1417fb3cf2c3cf2374c993b0a4e106b3
Reviewed-on: https://go-review.googlesource.com/6650Reviewed-by: Alan Donovan <adonovan@google.com>