iteratee 0.15.0

This is the first release for Cats 1.0.0-RC1. It removes the iteratee-fs2 artifact, which had previously provided a module for streaming in the context of fs2's Task type, which no longer exists in fs2 0.10 (it's been replaced by the IO in cats-effect, which will be more directly supported in upcoming versions of iteratee.io).

This release makes one change to behavior of the internal type Step: feeding a completed value now adds the new elements to the value's "leftovers". This fixes a bug in the tailRecM implementation for Iteratee (#219), but otherwise generally shouldn't affect user-facing code.