Alterations to object by label observers

1. Removes support for annotating the syntax tree with the last known
object to be observed for a label on that node. This is necessary for
syntax tree memoization. This also breaks a few Montage test cases that
may or may not be important: `serialization-spec` localizations unit
serializer serializes default message binding, and serializes data
binding, as well as `bindings-spec` should serialize a binding that
references external objects.
2. Adds support for `observeObjectByLabel` that has been useful in
experiments where the object in scope may change in response to panel
changes in a substitution.
3. Removes the alias `getComponentByLabel` because, in the context of
Montage serialization, labels may be applied to any value.