Stopped generating unnecessary ModIds for Dests.
Fixed a bug where we weren't processing nodes in the right order by switching
from a PriorityQueue to a TreeSet.
Added Datastore.removeMod for better encapsulation.
Fixed a bug in the last commit.
Added memoization to DoubleModList.reduce.
Created ModList(Node/Modifier/etc).
Merge remote-tracking branch 'upstream/master' into quicksort

Conflicts:
	core/src/main/scala/tbd/mod/DoubleModList.scala