Add user PING support (#346)

- Add `share::PingPong`, which can send `Ping`s, and poll for the `Pong`
  from the peer.

src/frame/ping.rs

@@ -14,6 +14,7 @@ pub struct Ping {
 // This was just 8 randomly generated bytes. We use something besides just
 // zeroes to distinguish this specific PING from any other.
 const SHUTDOWN_PAYLOAD: Payload = [0x0b, 0x7b, 0xa2, 0xf0, 0x8b, 0x9b, 0xfe, 0x54];
+const USER_PAYLOAD: Payload = [0x3b, 0x7c, 0xdb, 0x7a, 0x0b, 0x87, 0x16, 0xb4];
 
 impl Ping {
 
@@ -23,6 +24,12 @@ impl Ping {
     #[cfg(not(feature = "unstable"))]
     pub(crate) const SHUTDOWN: Payload = SHUTDOWN_PAYLOAD;
 
+    #[cfg(feature = "unstable")]
+    pub const USER: Payload = USER_PAYLOAD;
+
+    #[cfg(not(feature = "unstable"))]
+    pub(crate) const USER: Payload = USER_PAYLOAD;
+
     pub fn new(payload: Payload) -> Ping {
         Ping {
             ack: false,