Generic types and Serializable

[pvlugter]

The Serializable type is currently a union that includes any, making the whole type any so it's untyped. The any should probably be object instead, as indicated by the original jsdoc type (which converts Object to any).

Generics could be used for classes, especially those that have state that can extend Serializable: entities and replicated data types. Type parameters can have defaults, so the previous behaviour can also be supported in the same way.

[johanandren]

Is this completed with the three PRs merged @pvlugter or is there something more missing?

[pvlugter]

Still more to do: entities, handlers, contexts.

[pvlugter]

This will also affect codegen, which should use the typed versions instead of the generated workaround that it has currently. Probably worthwhile updating some codegen things at the same time, such as using Reply. Could also affect the generated testkit.js, which should really be part of the SDK directly.