- Initialize the `AnnotationStorage`-instance, on `PDFDocumentProxy`, lazily.
- Change the `AnnotationStorage` to use a `Map` internally, rather than a regular Object (simplifies the following points).
- Let `AnnotationStorage.getAll` return `null` when there's no data stored, to avoid unnecessary parsing on the worker-thread. This ought to "just work", since the worker-thread code *should* already handle the `!annotationStorage` case everywhere.
- Add a new `AnnotationStorage.size` getter, to be able to easily tell if there's any data stored.
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