Fix runtime crash as described on cython-devel.

__pyx_CyFunctionObject.defaults must be fully zeroed or a segfault or
memory corruption may occur if the object is visited by the garbage
collector before defaults is later populated. Uninitialized memory is
cast to PyObject* with undefined results:

  static int __Pyx_CyFunction_traverse(__pyx_CyFunctionObject *m, visitproc visit, void *arg)
  {
      ...
      if (m->defaults) {
          PyObject **pydefaults = __Pyx_CyFunction_Defaults(PyObject *, m);
          int i;
          for (i = 0; i < m->defaults_pyobjects; i++)
              Py_VISIT(pydefaults[i]);
                       ^^^^^^^^^^^^^

refactor analyse_types() and friends to work more like a transform by returning the node or a replacement

simple optimisation that statically maps keyword arguments to positional arguments when called signature is known

remove warning about missing 'const' support from docs and fix advice on declaring macros as 'enum' (which works only for ints)