Simplify the implementation of Active Model's type registry

Things like decorations, overrides, and priorities only matter for
Active Record, so the Active Model registry can be implemented much more
simply. At this point, I wonder if having Active Record's registry
inherit from Active Model's is even worth the trouble?

The Active Model class was also missing test cases, which have been
backfilled.

This removes the error when two types are registered with the same name,
but given that Active Model is meant to be significantly more generic, I
do not think this is an issue for now. If we want, we can raise an error
at the point that someone tries to register it.

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