Fixing 1591: Adding support for object literals in additionalProperties

[blast-hardcheese]

Partially resolves #1591

This is an initial pass, without modifying the deserializers.

What is supported as of this PR:

      properties:
        jwtTokensByRole:
          type: object
          additionalProperties:
            type: object
            properties:
              items:
                ...

The additionalProperties triggers emission of a Map[String, A] (eg: case class Bar(foo: Map[String, Bar.Baz])), as well as the nested definition for the Schema[_] literal at that position.

---

What needs to be done for full compliance with the spec (I've left a comment to this effect) is to introduce yet another intermediate class type that can hold the parameters that may come along with a MapSchema.

What still fails after this change:

      properties:
        jwtTokensByRole:
          type: object
          properties:  # Mixing `properties` and `additionalProperties` is still unsupported
            foo:
              type: string
          additionalProperties:
            type: object
            properties:
              items:
                ...