Add sync::watch::Sender::borrow method

[JOT85]

Motivation

The watch::Receiver type has the borrow method, to borrow the value last sent.

The watch::Sender also has access to this, but it isn't exposed. It would be nice to have this when passing around a Sender since it enables someone to check what has last been sent.

Solution

I've added an identical borrow method to sync::watch::Sender, along with some tests for the borrow method in general.

[Darksonn]

Hm, interesting. In any case, this isn't the most important method (that is send), so this shouldn't be at the top of the documentation.

[carllerche]

Thanks!

[carllerche]

@Darksonn good call. I will hold off merging

[JOT85]

Hm, interesting. In any case, this isn't the most important method (that is send), so this shouldn't be at the top of the documentation.

Thanks! Updated in b364216. I've placed it after send and before is_closed, I think that's a sensible place?

[Darksonn]

That is fine with me.