Made the output generated by Pyrex much more verbose with better context information.

Made the output generated by Pyrex much more verbose with better context information.

For example:

  /* "/Volumes/HOME/s/devel/sage-1/sage/matrix/matrix_generic_sparse.pyx":581
    x = set(v.keys()).intersection(set(w.keys()))
    a = 0
    for k in x:             # <<<<<<<<<<<<<<
        a = a + v[k]*w[k]
    return a
 */
  __pyx_3 = PyObject_GetIter(__pyx_v_x); if (!__pyx_3) {__pyx_filename = __pyx_f[0]; __pyx_lineno = 581; goto __pyx_L1;}
  for (;;) {