Adjust range_adjacent to support different canonicalization rules.

The original coding would not work for discrete ranges in which the
canonicalization rule is to produce symmetric boundaries (either [] or ()
style), as noted by Jeff Davis.  Florian Pflug pointed out that we could
fix that by invoking the canonicalization function to see if the range
"between" the two given ranges normalizes to empty.  This implementation
of Florian's idea is a tad slower than the original code, but only in the
case where there actually is a canonicalization function --- if not, it's
essentially the same logic as before.