- The default implementations of evalf(), diff(), normal() and expand() use

  map() where useful. This has the nice effect of having a more reasonable
  default behaviour for container functions (most of the evalfchildren() etc.
  stuff is gone).
- diff() works with non-commutative products (product rule) and no longer
  bails on indexed objects.
- added decomp_rational()
- added sqrfree_parfrac() which doesn't yet work in the general case and
  is unsupported