Stream as a name is taken, so we use Flow (which does not happen to be
in scope for every Scala program already). This also makes it clear that
this constructs just the the pipe, which needs to be activated to become
a Producer.
Then, the different language bindings need to live in different
packages, otherwise they would not be able to use the same name of the
central abstraction. The plan is to use scala_api, java_api and
java8_api, for starters.
