Added a "same_receiver" method to (Unbounded)Sender

[r3v2d0g]

Hello!
This was asked in #1588 and I wanted to make a first contribution 😄. As proposed by the issue's creator, I used Arc::eq_ptr to compare the inner field of SenderInner. I also added a partial_eq (though, I'm not really sure about its name) same_receiver test which compares a Sender with a clone of it when both are connected to the channel, when one of them is disconnected and when the channel is closed; it also compares two different channels' Senders.

[ghost]

In crossbeam-channel, we were considering adding the same impls but, after a long discussion, concluded it would be better to have methods that test whether senders/receivers are equivalent: crossbeam-rs/crossbeam#319

[r3v2d0g]

Oh, right, so I read the changes from crossbeam-rs/crossbeam#320 and it seems that the way it compares is similar to Arc::ptr_eq (in some cases it even is how it does it). So, if I understand correctly, removing the two impls and replacing them with a identical_to method would be OK?

[cramertj]

Perhaps fn same_receiver(&self, other: &Self) -> bool?

[cramertj]

Great, thanks!