More progress on usage parser.
More progress on usage parser.

I also did some pretty significant tweaking of the
representation. Resisting reference counting was
a mistake.

zsiciarz/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs,alexcrichton/docopt.rs,birkenfeld/docopt.rs,alexcrichton/docopt.rs,brandonson/docopt.rs