Make group-by preserve the ordering of the original list.
vectorof contract: add some tests and make flat variant subscribe to the val-first protocol
redex: fix stepper

This was exposed by the removal of the undefined value.

It's not clear if there was a bug lurking here or not.
The call to "move" was being bypassed on the initialization
of "pb", so the side effect of setting "set-visible?" wouldn't
have happened. Either that really didn't cause any problems,
or perhaps things looked strange briefly until the user did
something.
racket/unit: repairs related to signature extension

Fix signature extension so that the scope of identifiers in the
extended signature behaves the same as `open`.

Also, fix `struct` (or generally `define-values-for-export`)
in its interaction with `open` and `unit-from-context` (two
different bugs).
added non-terminal-gap-space