Classes are not rogue!

[vyzo]

This completes the MOP into the future, as classes are no longer rogue type descriptors; they are proper classes themselves, subtyping the gambit rtd.

As such, we drop the "type-descriptor" nomenclature and just call them classes and std has a fatality, misc/rtd is no longer useful; the accessors use ##structure-ref with the real class of classes, so they are safe.

At this point, we can safely say that MOP; Bootstrap, achieved.

Note This pr assumes https://github.com/gambit/gambit/pull/891 for complete correctness and future proofing; once that merges, we should update our gambit pin.

This will also allow us to provide safe rtd field accessors with type-{id,name,super,flags,fields} using a ##strcture-ref with ##type-type as the expected type.

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