+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?
this doc is (originally) wrong. It should be:

   * Note that a `ZipWith` with a specific name can only be used at one place (one vertex)
   * in the `FlowGraph`. Calling this method several times with the same name
   * returns instances that are `equal`.