Validation and serialization improvements

[confluence]

This PR:

• Adds a new InstanceOf validator which just checks the value against a type or tuple of types (using isinstance under the hood).
• Fixes the IterableOf validator to add a more specific error message if the value isn't iterable at all, and to make the description more readable
• Moves the macro-specific serialization logic from the serializer to the Macro object. Any custom object with a json method will now use that method for serialization.