add watch

[liufuyang]

I tried to add watch the same way as the broadcast side, but now I ended up with an error like this:

warning: variable does not need to be mutable
  --> tokio-stream/src/wrappers/watch.rs:40:18
   |
40 |     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
   |                  ----^^^^
   |                  |
   |                  help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

error[E0596]: cannot borrow data in a dereference of `Pin<&mut WatchStream<T>>` as mutable
  --> tokio-stream/src/wrappers/watch.rs:41:18
   |
41 |         Pin::new(&mut self.inner).poll_next(cx)
   |                  ^^^^^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `Pin<&mut WatchStream<T>>`

error: aborting due to previous error; 2 warnings emitted

Hopefully you can give a hand here? :)