feat: add uniq method to AsyncIterator

[jeswr]

This add the method .uniq the the AsyncIterator which captures a common pattern used with AsyncIterators of filtering elements in the iterator such that only unique elements are passed through.

Since this uses the .filter method directly this will be unaffected by #59

[coveralls]

Coverage remained the same at 100.0% when pulling 8e0d863 on feat/uniq into 358608f on main.

[RubenVerborgh]

Thanks a lot—releasing this now as v3.7.0.