[Pitch] Consider building out ScaleDegree structure

[jsbean]

Build out a ScaleDegree structure, with different representational affordances. The ScaleDegree might have an index (0, 1 ...), and depending on the context of the Scale (major, minor, etc.) it may be represented in different ways (I or i, etc.).

Such a structure could also encapsulate the logic of representing diminished / half-diminished, aug 6, sharp-5-flat-9 extensions.