Use NSData for sendToDevice

- In this gobind case, NSMutableData is not required to avoid an
  allocation/copy and may actually cause one

MobileLibrary/iOS/PsiphonTunnel/PsiphonTunnel/PsiphonTunnel.h

@@ -292,9 +292,9 @@ typedef NS_ENUM(NSInteger, PsiphonConnectionState)
  Implementing this method is *required* if whole device mode is enabled (and otherwise should not be implemented).
  The implementation of this must send the given packet to the device.
  @param packet  The data packet to send to the device.
- Swift: @code func send(toDevice packet: NSMutableData) @endcode
+ Swift: @code func send(toDevice packet: Data) @endcode
  */
-- (void)sendToDevice:(NSMutableData * _Nonnull)packet;
+- (void)sendToDevice:(NSData * _Nonnull)packet;
 
 @end
 

MobileLibrary/iOS/PsiphonTunnel/PsiphonTunnel/PsiphonTunnel.m

@@ -765,9 +765,9 @@
 }
 
 
-#pragma mark - GoPsiPacketTunnelDeviceWriter protocol implementation (private)
+#pragma mark - GoPsiPacketTunnelDeviceSender protocol implementation (private)
 
-- (void)sendToDevice:(NSMutableData *)packet {
+- (void)sendToDevice:(NSData *)packet {
     // The check to see if the delegate responds to this optional selector was
     // done when processing the config, so we're not going to do it again for every packet.
     [self.tunneledAppDelegate sendToDevice:packet];