[SpellPitch] Add Unspelled and Spelled protocols

[jsbean]

cc @bwetherfield.

Issue

We currently have a matrix of types which describe Pitch and Pitch.Class values and how they map into the Spelled universe.

Unspelled	Spelled
Pitch.Class	Pitch.Spelling
Pitch	SpelledPitch

For the purposes of PitchSpeller, it would be compelling to define Unspelled and Spelled protocols. This would generalize PitchSpeller from being concrete over (Pitch) -> SpelledPitch or (Pitch.Class) -> Pitch.Spelling to being generic over (Unspelled) -> Spelled.

Design proposal

Conform Pitch and Pitch.Class to the Unspelled protocol, and SpelledPitch and Pitch.Spelling to the Spelled protocol. We may need to add a unifying super protocol, from which each of the new protocols inherit.

I assume we will need to add some operation to these the super protocol that can map from the Unspelled to Spelled state given some context.

[bwetherfield]

This is great. Makes it much clearer to me how it might be possible to genericize PitchSpeller