[Duration] Implement more type-safe Rhythm.Leaf.Kind.Collection and Builder

[jsbean]

There is currently no way of ensuring that a .continuation(...) does not succeed an .absence.

Perhaps a good way to deal with this is similar to the Path.Builder.

extension Rhythm.Leaf.Kind {
    struct Collection { }
}
protocol AllowingStart { 
    func start(_ attack: Element) -> AllowingStart & AllowingContinuation & AllowingAbsence
    func start(_ attack: Element, _ release: Element) -> AllowingStart & AllowingAbsence
}
protocol AllowingContinuation { 
    func continuation(_ sustain: Element) -> AllowingStart & AllowingContinuation & AllowingAbsence
    // if a release element is provided, don't allow continuation
    func continuation(_ sustain: Element, _ release: Element) -> AllowingStart & AllowingAbsence
}
protocol AllowingAbsence { 
    // don't allow continuation after rest
    func absence() -> AllowingStart & AllowingAbsence
}
extension Rhythm.Leaf.Kind.Collection {
    final class Builder { 
         var storage: [Leaf.Kind] = []
    }
}
extension Rhythm.Leaf.Kind.Collection.Builder: 
    AllowingStart, 
    AllowingContinuation,
    AllowingAbsence 
{
    ...
}