add StreamingIteratorMut::flatten

[LorenzSchueler]

• add StreamingIteratorMut::flatten
• add Flatten
• add impl StreamingIterator for Flatten
• add impl StreamingIteratorMut for Flatten
• add test
• fix clippy warnings

[cuviper]

This is clever to build Flatten on StreamingIteratorMut! That gives it a kind of superpower that the regular FlatMap can't match with just identity, since this can iterate the borrowed inner iterator in-place.

[LorenzSchueler]

Thanks for the review. I agree with your suggestions. Currently I am on vacation but I will follow up on this once I get back.
(In case you want to merge this earlier feel free to apply the changes yourself.)

[LorenzSchueler]

I opened #28 for the clippy warnings

[cuviper]

I rebased and added a couple more fixes for the things below. If that looks good to you, then I think we're all set!

[LorenzSchueler]

looks good to me :)