Migrate to Swift 5

[1ec5]

Migrated to Swift 5, for now using the Swift 5.0 migrator in Xcode 10. The idea is to have as clean a slate as possible for the Objective-C obsolescence work in #2231.

Recap:

• Migrate to Swift 5.0
• Migrate to Swift 5.1 – too soon to require Xcode 11
• Fix switch exhaustiveness warnings

Blocked by #2206.

/cc @mapbox/navigation-ios

[1ec5]

The migration introduced a number of warnings:

/path/to/mapbox-navigation-ios/MapboxCoreNavigation/MBNavigator.swift:21:9: Switch covers known cases, but 'MBRouteState' may have additional unknown values
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/MBNavigator.swift:21:9: Handle unknown values using "@unknown default"
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/EventDetails.swift:354:9: Switch covers known cases, but 'UIApplication.State' may have additional unknown values, possibly added in future versions
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/EventDetails.swift:354:9: Handle unknown values using "@unknown default"
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/DistanceFormatter.swift:39:9: Switch covers known cases, but 'LengthFormatter.Unit' may have additional unknown values, possibly added in future versions
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/DistanceFormatter.swift:39:9: Handle unknown values using "@unknown default"
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/DistanceFormatter.swift:70:13: Switch covers known cases, but 'LengthFormatter.Unit' may have additional unknown values, possibly added in future versions
/path/to/mapbox-navigation-ios/MapboxCoreNavigation/DistanceFormatter.swift:70:13: Handle unknown values using "@unknown default"
/path/to/mapbox-navigation-ios/MapboxNavigation/StatusView.swift:118:13: Switch covers known cases, but 'UIUserInterfaceLayoutDirection' may have additional unknown values, possibly added in future versions
/path/to/mapbox-navigation-ios/MapboxNavigation/StatusView.swift:118:13: Handle unknown values using "@unknown default"
/path/to/mapbox-navigation-ios/MapboxNavigation/Style.swift:484:13: Switch covers known cases, but 'UIUserInterfaceLayoutDirection' may have additional unknown values, possibly added in future versions
/path/to/mapbox-navigation-ios/MapboxNavigation/Style.swift:484:13: Handle unknown values using "@unknown default"

Some of these issues can be addressed by removing LengthFormatter.Unit usage in favor of Measurement<UnitLength> as part of either #2206 or #2114. As for switch exhaustiveness warnings, we can accept a fatalError() on some of them, but others can probably be turned into simple if-else statements.

[1ec5]

Some of the builds are failing because Swift 5 isn’t supported in the Xcode 10.1 / iOS 9.3 toolchain. #2206 would remove this build in favor of an Xcode 11 / iOS 13 toolchain.