Feature request: `take_while_distinct()` adapter

[ronnodas]

This would be to unique() what take_while() is to filter(): iter.take_while_distinct() would yield items from iter until it encounters the first duplicate, at which point it yields None. I've needed this a few times and while it's not hard to implement myself, or even write an inline version, itertools also seems like a good place for it.

The type bounds should be the same as unique():

Self::Item : Clone + Eq + Hash

and the implementation would be equivalent to:

let mut seen = HashSet::new();
iter.take_while(|item| seen.insert(item.clone()))

I'm not particularly attached to the name, take_while_unique() or while_distinct() seem reasonable too.

If this is desirable, I can write a PR.