Remaining are:
- parts of unit200 that Matthew plans to remove.
- the `mzscheme` implementation itself.
The implementation of `mzscheme` has been moved
to the `mzscheme` collection (from the `racket` and
`scheme` collections). The `scheme/mzscheme`
language, which was undocumented, has been removed.
This is slightly backwards-incompatible, because
the `xform` handling of precompiled headers now
evaluates code in a `racket/base`-like namespace,
instead of in a `mzscheme`-like namespace.
original commit: d54c1e4e4942c26dcbaaebcc43d5c92d507a8112
