Rollup patch with a bunch of small fixes in it.  Can I get a quick turnaround review on this?  I'd like it to go in before today's beta is cut.  Definitely the core change has to go in, but that's uncontroversial.

  Core:
  * _PyType_GetDocFromInternalDoc() and
    _PyType_GetTextSignatureFromInternalDoc() are now marked
     Py_LIMITED_API.

  Tools:
  * The default filename for the "file" preset is now:
     "clinic/{filename}.h".

  * The "class" directive syntax has been extended with two new required
    arguments: "typedef" and "type_object".  "typedef" should be the
    equivalent C type for this Python class, presented as a
    pointer-to-type.  "type_object" should be an expression that
    evaluates to a PyTypeObject * for this Python type.  Example:
        class dict "PyDictObject *" "&PyDict_Type"
    This was necessary for the next item.

  * If __new__ or __init__ functions did not use kwargs (or args), the
    PyArg_NoKeywords (or PyArg_NoPositional) calls generated are only
    run if the type object is an exact match.  (Otherwise subclasses
    will not be allowed to start accepting those kinds of arguments.)

  * Argument Clinic now fails if you have required parameters after
    optional parameters.

  * Converters now have a new template they can inject code into:
    it's called "modifiers", and code put there is run in the parsing
    function after argument parsing but before the call to the impl.
    "modifiers" is intended for converters that may want dynamic
    default values.  An example string you might add:
        "if ({name} == NULL)\n    name = time(NULL);"