cl-num-utils and utf8

I find Unicode math symbols very useful for mixing a bit of math and text — even though they don't even begin to approach the flexibility of LaTeX, all editors and browsers should be able to render them by now. When writing Lisp code, I only use them in docstrings. For example, cl-num-utils contains comments like this:

(defclass interval/infinite-left ()
  ()
  (:documentation "Left endpoint is -∞."))

Anton Vodonosov has recently reported that this causes a problem when the default external format is not utf8, also suggesting that I include an :encoding clause in the system definition. I have just pushed this change and I hope that this fixes the issue for everyone.

I appreciate the work Anton and others have put into cl-test-grid. I usually use a single implementation (the latest released SBCL), but I strive to make my code compatible with all implementations. This is much easier to achieve with cl-test-grid.