Add field.primary and field.references

[Dr. Simon Harrer]

models:
  orders:
    fields:
      order_id:
        type: text
        primary: true
  line_items:
    fields:
       line_item_id:
         type: bigint
         primary: true
       order_id:
         type: text
         references: orders.order_id

[Dr. Simon Harrer]

This is approved.

[Dr. Simon Harrer]

primary has been added to the spec.

[Dr. Simon Harrer]

references is now part of the standard