Metafont combinators

This is a proposal for Metafont combinators, potentially a nicer interface than the text parser or the quasiquoter we've discussed.  I'm pretty happy with the three new operators, but not terribly happy with the ways of describing paths, especially directions.  Any better ideas?

See also the tutorial on these combinators: https://github.com/diagrams/diagrams-doc/blob/metafont/doc/metafont.rst

I'll submit a separate pull request for that after this one is closed, but I figure the examples may help when reviewing this.