StreamData: data generation and property testing for Elixir

Since it is being considered for inclusion in Elixir itself then it should be written in Elixir - so it fits with everything else provided by the language and with a matching license. Also note that we decided to build on top of the Stream functionality we provide as part of Elixir, so they work both as data generators and as property testing - they are related but independent. However, the other solutions are more mature and they also provide state checking - something we don’t plan to explore at the moment.