0.31

• Peter Sewell (+ Thibaut Pérami): Improve experimental pretty-printing back-end
• Peter Sewell: Some fixes in Tex generation
• Peter Sewell: Add ocamllex-from-string hom to tweak the lexing of a metavar
• Peter Sewell + Thibaut Pérami: Add menhir-start-type hom to specify the top level type of a menhir parser