Created an IndexValueStore class which can store any object and return
an integer index to its hash map.
Linear data is now stored in ve.dm.LinearData instances. Two subclasses
for element and meta data contain methods specific to those data types
(ElementLinearData and MetaLinearData).
The static methods in ve.dm.Document that inspected data at a given
offset are now instance methods of ve.dm.ElementLinearData.
AnnotationSets (which are no longer OrderedHashSets) have been moved
to /dm and also have to be instantiated with a pointer the store.
Bug: 46320
Change-Id: I249a5d48726093d1cb3e36351893f4bff85f52e2
