remove testing hack
repair interaction of taints and submodule expansion
catch up on documentation for some library changes

The library changes were made as part of the package reoganization.
.gitignore fixups
Move `racket/sandbox` etc to `sandbox-lib` package.
Move `errortrace` collection and documentation to new packages.
Move srfi/4 out of the core.
Move various data structures to `data-lib` pkg.
Move `help` collection out of the core.
Move unstable/contract and unstable/options to their own packages.
Move `compatibility`, pconvert, and mutable pairs to new packages.
Move much of `mzlib` to `compatibility-lib` package.
Move `rackunit` to the `rackunit-lib` package.

The `rackunit/log` utility stays in the core, because it's
used by `raco test` and very small.
Typo in comment
scribble: add tests for `traverse-element' and `traverse-block'
switch "config.rktd" dirs back to "collects"-relative

Making them "etc"-relative intercts badly with redirecting
a configuration to override just a few values --- such as
the links path, which is what `make core' needs.
Use racket/cmdline instead of mzlib/cmdline

This should fix the build
rewrite mzlib/contract to racket/contract
adjust mzlib/contract so that it doesn't require any racket/contract/private stuff
move mzlib/contract into compatibility-lib