Tag model entities with their "type"

this aims at preventing constant usage of isinstance() based dispatch code when writing generic code handling model entities.

For example, the "object_type" argument of JournalWriter.write_addition() has become superflous now we only pass model entities, etc.

This idea comes olasd's reading of mypy doc:

https://mypy.readthedocs.io/en/latest/literal_types.html#tagged-unions

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Migrated from D3152 (view on Phabricator)