[racket-dev] Things we could move out of the core

While moving some files around between packages, I realized that there
are a number of things that could be moved out of the core and into
packages.  Here's a partial list of things that I think are not needed
at all by the rest of the core:

- racket/sandbox and the rest of the sandbox code
- data/{union-find, interval-map, splay-tree, skip-list, order, gvector, heap}
- srfi/4
- unstable/{options, contract}
- errortrace (once sandbox is removed)
- compatibility/*
- mzlib/{pconvert, class100, serialize, thread, transcr}
- a bunch of other parts of mzlib that are just reprovides

The following are libraries that could be removed with (I believe)
small changes to the rest of the core:

- rackunit
- srfi/13
- mzlib/unit200 (maybe, this might be hard)
- mzlib/kw

If this seems like a good idea to people, I'm happy to go ahead and
start doing the work.

Sam