Simplify stream-unfolds further, based on Mark H. Weaver's feedback.

* module/srfi/srfi-41.scm (stream-unfolds): Don't use a promise to
  hold the result of the generator call. This means the generator
  will always get called once more than necessary, but it does make
  the code somewhat simpler, and the saved generator call probably
  wasn't worth the added code complexity.