fix: improve handling of allOf / anyOf / oneOf (#410)

- introduces a new intermediate type for `intersection` and `union`,
making handling of compositions in schema/type builder easier to reason
about
  - though `oneOf` is being treated as equivalent to `anyOf` for now
- improves normalization process treat the base schema as part of an
`allOf`/`intersection` when an object schema is part of a composite,
fixing previous information loss
- improves handling of `oneOf` / `anyOf` to support "looking up"
property definitions from the parent object
  - this is still limited, eg: won't traverse up multiple-levels

after this change it appears `EmptyObject` is no-longer emitted by any
of the integration-tests - which is generally a good sign

Author: mnahkies