Add a has_changed function to watch::Receiver

[elichai]

Motivation

I'm using sync::watch to tell a bunch of workers to do some work, and once in a while they check to see what's the latest job, and if there's no new job they should keep doing what they're already doing, and currently the only way to do that is via rex.changed().now_or_never().is_some() which has some Waker overhead (see asm in https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=8fdf2f1bf2b6b16827b106dc13ef5895)

Solution

Add a has_changed method to check if the value had changed.

Alternatives

1. This method doesn't change the version that the receiver currently holds, this is the only function in the API that observes the version without changing it, should it maybe also change it? (that comes at the cost of being a &mut self function).
2. Implement a try_changed function that doesn't block if the value hasn't changed.

[Darksonn]

Thanks.