Alterations to object by label observers

[kriskowal]

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.

[aadsm]

Just being curious, where were we using this information?

[kriskowal]

stringify is/was rigged to use components.getLabelForObject or components.getObjectLabel on syntax.component to consult a deserializer to rewrite the label based on which serialization it was being used in. This, if you recall, was necessary for making round trips through the serialization system.

[kriskowal]

@aadsm We should figure out whether we’re still using this feature.

[kriskowal]

I will punt this issue to the v2 branch and merge myself. I’ll make a note in the change log to make it less mysterious why things break if we’re still depending on syntax annotation at run-time in MontageJS.