Add OptionSet macro to help build option sets

[DougGregor]

Add an OptionSet macro that can be used like this:

@OptionSet
struct ShippingOptions {
  private enum Options: Int {
    case nextDay
    case secondDay
    case priority
    case standard
  }
}

to build a proper option set.

[DougGregor]

This requires the March 2nd snapshot. Merging now that that's available.