Add a .unique() method to Iterator.

[Wilfred]

I've found it very useful to be able to discard duplicates from iterators. I think this would be a useful addition to itertools, what do you think?

[bluss]

Hi, that sounds useful. What do you think about this:

• It should be in the itertools trait
• Use a single struct, not a struct and a trait.
• Add it in the form of .unique_by with the closure giving the key
• You're welcome to put it in a new module, but not a public submodule.

[bluss]

This was superseded by PR #37