Move most of the `net` collection out of the core.

- Most units and signatures from the `net` collection
  are now in `compatibility-lib`.
- Most of the actual libraries are in the `net-lib`
  package.
Copy many fewer files for xform.
Fix uses of 'mzscheme' in comments.
Remove most uses of `mzscheme` in the core.

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.
Move most of the `compiler` collection to `compiler-lib`.