Replace NEKit dependency with reduced subset

GlassVPN/zhuhaow-NEKit/IPStack/IPStackProtocol.swift

@@ -0,0 +1,34 @@
+import Foundation
+
+/// The protocol defines an IP stack.
+public protocol IPStackProtocol: class {
+    /**
+     Input a packet into the stack.
+
+     - parameter packet:  The IP packet.
+     - parameter version: The version of the IP packet, i.e., AF_INET, AF_INET6.
+
+     - returns: If the stack takes in this packet. If the packet is taken in, then it won't be processed by other IP stacks.
+     */
+    func input(packet: Data, version: NSNumber?) -> Bool
+
+    /// This is called when this stack decided to output some IP packet. This is set automatically when the stack is registered to some interface.
+    ///
+    /// The parameter is the safe as the `inputPacket`.
+    ///
+    /// - note: This block is thread-safe.
+    var outputFunc: (([Data], [NSNumber]) -> Void)! { get set }
+
+    func start()
+    
+    /**
+     Stop the stack from running.
+
+     This is called when the interface this stack is registered to stop to processing packets and will be released soon.
+     */
+    func stop()
+}
+
+extension IPStackProtocol {
+    public func stop() {}
+}