Mullein

Copperbox revision 693.

First attempt at symbolic durations in the Mullein2 branch.

Durations that can be represented in scores are fairly limited - in LilyPond I think their lengths span from longa to 128th and they can be dotted and double-dotted (maybe triple dotted or more I haven't really looked). Representing them as rationals gives plenty of opportunity to create unprintable durations and allows numeric operations (+,-,etc.) that aren't defined. As I'm no longer trying to be compatible with Haskore it seems worthwhile to try representing them symbolically.

This first draft still represents durations internally as rationals - I think I ought to go further and represent then as an algebraic type.