Add deftransform to convert (log x 2) and (log x 10) to log2 and

%log10. This better than using the definition (/ (log x) (log base)).
This also allows exact answer for 2^n and 10^n for appropriate n.


 * src/compiler/float-tran.lisp:
   * Add deftransforms to convert (log x 2) and (log x 10) to
     kernel::log2 and kernel:%log10 
 * tests/float-tran.lisp:
   * Add tests to check the transforms are done, or not done, as
     appropriate.