Implement flux generator

[clue]

Right now, this project only implements the flux Transformer which allows you to "transform" a stream from one form to another by passing it through a promise-based API.

This package should also provide a flux Generator which allows you to "generate" a stream of data from a promise-based API. A common use case would be to generate a stream of "user" objects from a database client that always fetches X users at once.

By passing this stream into the existing Transformer, it would automatically take care of throttling the Generator to pause fetching new entries when the concurrency limit is reached and will automatically resume fetching once the outstanding operations have been fulfilled. It could thus effectively be used a form of a lightweight message queue.