Add all_equal

[phimuemue]

This refers to #175. I think going with dedup is a convenient, simple solution.

[bluss]

Ok, it's not always easy to decide, but I think this method should use &mut self and no Self: Sized, just like all. What do you think?

[bluss]

By the way, the method should be sorted among the methods, not the adaptors, so it needs to be moved down. next_tuple is the first non-adaptor in the trait.

[phimuemue]

I think we could use &mut self, but when I reconsidered the problem I found another issue that I think is possibly even more important: Do you think it would be wise to return Option<Item> instead of bool so that the users have the possibility to get a value if all are considered equal (and None if they are not equal)?

[bluss]

That's a good extension. But we need to find a new name of it then!

Hehe, I think of a good feature as being a combination of a natural name and a wanted functionality. We need both for it to be used.

[phimuemue]

I re-checked with cargo +nightly test on my machine and obtained test result: ok. 74 passed; 0 failed; 5 ignored; 0 measured as output.

I think the travis failure is related to rust-lang/rustup#1062.

[bluss]

Great, thank you!