Implement TreeSet#descendingIterator
Implement TreeSet#descendingIterator

If need be, this functionality can be sped up by implementing a
descending iterator on the tree without copying it into an ArrayList.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

marcinolawski/avian,bigfatbrowncat/avian-pack.avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,minor-jason/avian,MaartenR/avian,getlantern/avian,bgould/avian,MaartenR/avian,joshuawarner32/avian,marcinolawski/avian,bgould/avian,lwahlmeier/avian,joshuawarner32/avian,lwahlmeier/avian,dicej/avian,marcinolawski/avian,marcinolawski/avian,ucdseniordesign/avian,dicej/avian,getlantern/avian,lwahlmeier/avian,dicej/avian,minor-jason/avian,ucdseniordesign/avian,getlantern/avian,bgould/avian,bigfatbrowncat/avian-pack.avian,joshuawarner32/avian,getlantern/avian,ucdseniordesign/avian,dicej/avian,lwahlmeier/avian,lostdj/avian,bgould/avian,minor-jason/avian,MaartenR/avian,joshuawarner32/avian,ucdseniordesign/avian,lostdj/avian,MaartenR/avian,minor-jason/avian,lostdj/avian,bigfatbrowncat/avian-pack.avian