[plt-scheme] new require-spec: all-except-from

This would be really handy:

(require "foo.ss")
(require (all-except-from "bar.ss" "foo.ss"))

This would require everything from "foo.ss", then everything from
"bar.ss" that wasn't already imported from "foo.ss".  This would be
much nicer than using `all-except' and having to enumerate all the
imports from "bar.ss" that conflict with "foo.ss".

Is this doable?

--dougo at place.org