- power::eval(): for the case (b_n/b_d)^(e_n/e_d) we now check separately

  if we can compute numerator and denominator.  This allows us to eval
  (3/8)^(1/3) to 1/2*3^(1/3) instead of holding it.  For square roots this
  is still less clever than what MapleV does, but then again that system
  has the funny goof not to touch 8^(1/3) where we immediately eval() to
  plain 2; fun, fun, fun...
  Oh, and a little memory hole has been squished along the way, too.