+str - adds ZipWith and makes Zip use that to create tuples/pairs

@patriknw @ktoso @2m @bantonsson, do we want this one? Or is Flexi replacing all the things?
yes, it's an identifying name, can be used when passing vertices between partial flow graph construction.

I think the original documentation of these methods have been messed up in Zip. See the two `apply` methods of `scaladsl.Merge` for how it was intended to be.