sync: add same_channel method to watch::Receiver

[ahrens]

Is your feature request related to a problem? Please describe.
I'd like to determine if two watch::Receivers are part of the same channel (i.e. will receive values from the same watch::Sender.

Specifically, I'd like to have a HashMap<K, watch::Receiver<V>>, and then later be able to check if my entry is still there, or someone else has removed it (and potentially replaced it with a Receiver for a different Sender).

Describe the solution you'd like
Add a method similar to:

impl<T> Receiver<T> {
    pub fn same_channel(&self, other: &Self) -> bool {
        Arc::ptr_eq(&self.shared, &other.shared)
    }
} 

Describe alternatives you've considered
The solution I have today is to use a HashMap<K, Arc<watch::Sender<V>> and then use Arc::ptr_eq() to check if the Sender is mine. The disadvantage is that there's an additional heap allocation/indirection for the Arc, and I need to ensure that no extraneous clones of the Arc are created.

Additional context
I noticed that @zh-jq opened #3532 to add mpsc::Sender::same_channel() and @Darksonn reviewed it. This is similar functionality.

I'd be willing to try to implement/upstream this, if it's likely to be accepted.

[Darksonn]

Adding this seems fine to me.