Migrate type hierarchy to use Rubydex

[vinistock]

Motivation

This PR migrates type hierarchy to use Rubydex.

Implementation

I moved all constant resolution to the prepare type hierarchy request, which pays the price once for resolving and then forwards the fully qualified name to super/sub types through the flexible data attribute. I also ensured that users can request singleton hierarchy.

The super types request is considerably simpler because the fully qualified name is always received through the data attribute.

The rest of the implementation ensures that we are handling singleton classes correctly because of their implicit nature. For example:

class Foo
  class << self
  end
end

The Foo::<Foo> declaration inherits from Object::<Object>, which exists in the graph, but there's no definition to back it up (it's implicit). In those cases, we need to find the definition of the original attached object, so that we can show something useful to the user.

Automated Tests

I added a bunch more tests verifying that we behave correctly under a variety of conditions.

[Morriar]

We delete this: Currently only supports supertypes due to a limitation of the index. but it doesn't look like we support subtypes yet?

[vinistock]

We don't, but it's not because of indexing limitations. Also, I think the absence of the subtypes request (which makes it not appear in the documentation) is enough to communicate that we don't yet support it.

This comment is likely to be forgotten once we add it.