Fix to handle (log 3/2 2) and (log -3/2 2).  Test added.

 * src/code/irrat.lisp:
   * Fix LOG2 to handle the case of negative rational numbers.
   * Fix LOG of rational to a rational base. We want to convert the
     log of the number to a single float because log2 isn't intended
     to handle single-float type.

 * tests/irrat.lisp:
   * Add test for log of rational to rational base.
   * Add some additional tests for dd-%log2.