This was necessary because Cargo recently deprecated the "overrides"
keyword in favor of "package", and Criterion changed its interface.
This commit also now lists more configurable keys, so if they aren't set
to their defaults the user will be able to find them. This includes the
roll key, which wasn't listed earlier and is different on Windows and
Mac. It does *not* include the respawn key because that key is already
shown when you die.
