runtime return type of `schema` should match its declaration

[tom-thorne]

We've been adding our own library wrapper around model in order to assert validation levels and extend the model with new functions. As a result, we are relying on the return type of schema.

However, our code fails at runtime since the actual type returned from schema isn't a [SchemaType<TProperties, TOptions>, TOptions]. It is instead an object of some sort. Looking at the project source, this appears to be by design.

It would be better (in my opinion) if the return type of schema matched its own declaration, and the fixup to the internal runtime type occurred within model