Mullein M2

Copperbox revision 1127

I've partially revised the syntax so that Bars and CExprs (tuplets | beam groups | atomic glyphs) are parametric only on a single parameter rather than three. Being parametric on annotation, pitch and duration seemed more appropriate when I was experimenting with KURE but now I think it is a burden instead.

Also I think having an AExpr for a single glyph or grace notes is not great. Originally I thought this was an improvement on Mullein which had grace notes as one of the constructors on the glyph type (I didn't like one of the glyph constructors having one-or-more pitches and durations when all the other constructors had zero-or-one pitch and durations). Unfortunately having AExpr's (single glyph | grace notes) is making bracketing difficult - it is much harder to account for BeamExtremity in two places in the syntax tree, rather than only on 'glyphs'.

I'll remove AExprs in the next commit, for the moment I want to checkpoint them.