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).
Reverted all changes, added Mod2, ReadN