Move `master` block outside of field/model definition

[SamProtas]

As more ways to "source" a model generator have been added, it's become clearer that the "source" and the model definition itself don't need to be so intertwined. In some cases this specifically doesn't make much sense.

Proposed future layout:

{ "model_name": { "master": { }, "model": { "field_1": { }, "field_2": ... } }