Fix some issues with log2 and log10 of big rationals.

 * src/code/irrat.lisp:
   * Add log10 function to handle the computation of the base 10 log
     of big rationals that might not fit in a double-float.
   * Fix some issues where CL:LOG wasn't handling logs of big
     rationals. (A regression).
 * tests/trac.lisp:
   * Update trac.8 test to include logs base 10.