Issue Description:

Currently, to signal a success of type Result<Void, Swift.Error>, we need to use:

.success(())

Combine, for example, special-cases Subject for when the Output is of type Void:

extension Subject where Self.Output == Void {
    /// Signals subscribers.
    public func send()
}

A similar extension could be added for `Result` so that we can use .success(()) instead:

extension Result where Success == Void {
    static func success() -> Self {
        return .success(())
    }
}
 

I'd be happy to contribute a patch if someone can point me to the proper PR policy.