Add support for global "dictionaries"

[skunkiferous]

Class-name-to-ID mapping is just one case of global "dictionary". Enums are another. We should build-in support for global dictionaries, where some value (String or otherwise) is mapped to an ID. Then we can refactor the classes to use it too.

[skunkiferous]

The simplest way of mapping IDs to cached/static object instances, specified by the schema, would be in fact to have one single unified dictionary. Whether or not classes should be in it is unclear, but I think it would work if classes were in it too. The reason to use a single dictionary is simple: it's because we don't need any additional meta-data anywhere. If we use the positive integer values to represent previously serialized instances on this stream (with or without "0"), then we could use negative IDs for objects coming from the global dictionary (or maybe the other way around, if that would be more efficient). Like this, we could use static cached instance anywhere, and we would recognize them without any additional data, just because of the sign of the ID. To build this dictionary, we would need one huge hashmap at schema definition time, but since the implementation of equals for any object should handle being compared to objects of other types, there should be no problem mixing objects of different types in the same hashmap.