Improve recursive Flow/TS types for gen()

[leebyron]

The top level gen() function (#30) allows deeply nested shapes that contain generators in any leaf position. This is also the function used to convert any value expected to be a generator to a generator. The type signatures of the functions in this library should be aware of this nested behavior, however that doesn't currently seem possible in either tool.

I'm keeping this task open to track the desire to improve those types.