cython-mode.el: fix and improve font-locking

* fix font-locking of keyword regexps (matcher was referring to group
  that ceased to exist after 0a7e8d6047f3fdd5a733ed9ed3baa60dabf612be)

* fix adding customized font-lock-keywords

  The issue with old code was that it changed an element of a list that
  was compiled into "python-mode" function definition.  This had an
  unfortunate side effect of propagating cython-specific keywords into
  vanilla python buffers.

* fontify new types in ctypedef statements

* add more keywords ("cppclass", "new", "del", etc.)

* add more builtin types ("Py_UNICODE", "ptrdiff_t", etc.)

* fix font-locking of "except?"

* fontify types with font-lock-type-face, not py-class-name-face