Add streaming generators

[DalekBaldwin]

Streaming generators would return a function that the test body can call to get as many times as it needs to get the values as it needs.

https://hypothesis.readthedocs.org/en/latest/data.html#infinite-streams

The generation history could be stored as a list paired with a function that iteratively produces values from it so that the list could be shrunk/saved as a regression case.

These would probably only make sense as top-level generators, not as subgenerators of some other generator.