Remove Clone requirement from unique

[gilhooleyd]

This trait requirement appears to no longer be needed.

[Philippe-Cholet]

The item has to implement clone for Unique to be an iterator

Iterating over a Unique still requires Clone.

[jswrenn]

@Philippe-Cholet's right. @gilhooleyd, could you submit a PR adding the Clone bound to the Unique definition:

itertools/src/unique_impl.rs

Lines 163 to 170 in 4d2bd4e

	/// An iterator adapter to filter out duplicate elements.
	///
	/// See [`.unique()`](crate::Itertools::unique) for more information.
	#[derive(Clone)]
	#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
	pub struct Unique<I: Iterator> {
	iter: UniqueBy<I, I::Item, ()>,
	}

[gilhooleyd]

Thanks for the quick feedback! I was surprised to see that cargo test passed with this change, but it looks like there's no test code that calls unique and then iterates over it. Adding that test and then trying to remove Clone further I see that the header comments are correct (Unique does clone each item to place it in the hashmap to look for duplicates).

I'll submit another PR to add a test and the trait