Integrate os_signpost for profiling

[kean]

Integrate os_signpost and add an option to enable it:

/// `false` by default. If `true`, enables `os_signpost` logging for
/// measuring performance. You can visually see all the performance
/// metrics in `os_signpost` Instrument. For more information see
/// https://developer.apple.com/documentation/os/logging and
/// https://developer.apple.com/videos/play/wwdc2018/405/.
public static var isSignpostLoggingEnabled = false