Introduced the ve.AnnotationSet class to manage sets of annotations. This
is a generalization of ve.OrderedHashSet, a class that manages a set
using an array and an object keyed by hash.
Converted everything that stores, tracks or passes around annotations to
use ve.AnnotationSet. In particular, this means the linear model now
contains AnnotationSets instead of hash-keyed objects.
This allows us to maintain the order of annotations in the linear model,
and will help fix bugs with annotation ordering and splitting.
Change-Id: I50975b0a95f4cc33017a0b59fdede9ed1eff0124