Allow `clojure.core.async/to-chan` accept an optional transducer that gets passed to the newly created channel.
Why? With transducer passed directly in `to-chan` there is no need to use `pipe` or `onto-chan` with an explicitly created a channel that has transducer attached.
This is my first patch for a Clojure project. Please let me know if something was wrong / missing when creating the ticket.