Metafont cleanup

I think the Metafont code is in a state where other people could use it.  Please try it out, find what I've missed.

This branch includes:
a) an implementation of the path solver described in The MetaFontBook and mf.web, resulting in Diagrams Trails
b) a Parsec parser implementing the string syntax used by MetaFont
c) a few tests producing SVGs that match diagrams in the MetaFontBook

Things I want to add, but I don't think prevent merging:
a) a proper quasiquoter with variable interpolation
b) a tutorial

Compared to the metafont branch, this branch:
a) works
b) is rebased onto current diagrams-contrib
c) does not have a separate commit for every off-by-one or sign- error I've found in the last two weeks :)

All these matrices of Doubles representing angles make me appreciate the Diagrams type system!