Issue Description:

There is a strong precedent from other programming languages that have optional types to include method named filter that conditionally maps the optional to nil. The implementation is trivial:

extension Optional {
    func filter(_ predicate: (Wrapped) throws -> Bool) rethrows -> Optional {
        return try flatMap { try predicate($0) ? self : nil }
    }
}

Swift is currently missing this method. When pitched in Swift Evolution Forums, this method was met with no opposition. This looks like a clear omission without any downsides, that should be easy to add into Swift standard library without the need to go through lengthy Swift Evolution process.