[SpelledPitch] Consider renaming Pitch.Spelling to PitchSpelling

[jsbean]

When one uses Pitch.Spelling downstream, they need to import not only the SpelledPitch module, but also the Pitch module.

Instead, if we rename Pitch.Spelling to PitchSpelling, and pull Pitch.Spelling out from under the extension of Pitch, someone can use it at the appropriate level of abstraction.

It is worth auditing how much breaks downstream (though in the end it may be for the better).

[jsbean]

Of course, we can make Pitch.Spelling just a typealias, and add a deprecation warning:

extension Pitch {
    public typealias Spelling = PitchSpelling
}