Use Swift 4.1's ability to wrap directives around switch cases

[ZevEisenberg]

https://bugs.swift.org/browse/SR-2 was resolved in Swift 4.1.

Affects this code in StringStyle+Part.swift:

        case let .emphasis(emphasis):
            self.emphasis = emphasis
        default:
            // interaction between `#if` and `switch` is disappointing. This case
            // is in `default:` to remove a warning that default won't be accessed
            // on some platforms.
            switch stylePart {
            case let .hyphenationFactor(hyphenationFactor):
                self.hyphenationFactor = hyphenationFactor
            default:
                #if os(iOS) || os(tvOS) || os(watchOS)
                    switch stylePart {
                    case let .speaksPunctuation(speaksPunctuation):
                        self.speaksPunctuation = speaksPunctuation
                        return