Refactor model validation

[DanielSchaffer]

The current validation scheme actually does two jobs:

• "reconstituting" a model from a "lesser" state
• "lesser" meaning plain JSON, a database representation, etc
• converts JS primitives into Dandi types (e.g. Uuid, Url, etc)
• converts complex objects into classes as determined by decorator

The goals of this change are:

• decouple the "reconstitution" step from the validation
• support for alternate discovery of property values (e.g. getting embedded values from HAL resources from the _embedded property)

This most likely means moving the "reconstitution" logic into @dandi/model, which will not be a problem after #12. Alternatively, use a new package (e.g. @dandi/model-builder)