This adds very basic (and super hacky) support for circular types to the
Ruby compiler, which should be _just_ enough to allow us to continue
work on the self-hosting compiler. Circular types can arise as follows:

    object TypeParameter {
      @required_traits: Array!(TraitType)
    }

    object TraitType {
      @type_parameters: Array!(TypeParameter)
    }

Without support for circular types code like this won't compile.

The changes needed to make this work are a total hack, and in some cases
will lead to some duplicate work being done (with the results being
discarded). Since this code will only stick around briefly, this is fine
for now.