Add unwrap_match! macro which is a general unwrap for enums

[L-as]

example:

	enum E {
		A(u32),
		B,
	}

	let e = E::A(2);
	assert_eq!(unwrap_match!(e, E::A(i) => i), 2);

I realise that you might not accept this, since it is not a part of the RFC, however I believe that this is a very useful feature.

[L-as]

I am not sure whether the formatting is correct, but I've used 4 spaces for indentation and I've made sure to not have any extraneous whitespace.

[SimonSapin]

I’m not very interested in expanding this crate. Consider publishing your own? And I’m not sure which RFC you mean, rust-lang/rfcs#163 was closed four years ago.

[L-as]

Sure, I'll close this pull request and make my own crate.

[SOF3]

@laaas did you eventually release such a crate?

[L-as]

@SOF3 https://docs.rs/matches2/1.1.0/matches2/

If something isn't to your liking, feel free to make a PR (or I can transfer ownership). Looking back at it, the thing I regret the most is the name probably.