Make identifiers str (not str8) objects throughout.

This affects the parser, various object implementations,
and all places that put identifiers into C string literals.

In testing, a number of crashes occurred as code would
fail when the recursion limit was reached (such as the
Unicode interning dictionary having key/value pairs where
key is not value). To solve these, I added an overflowed
flag, which allows for 50 more recursions after the
limit was reached and the exception was raised, and
a recursion_critical flag, which indicates that recursion
absolutely must be allowed, i.e. that a certain call
must not cause a stack overflow exception.

There are still some places where both str and str8 are
accepted as identifiers; these should eventually be
removed.

Python/pythonrun.c

@@ -1132,6 +1132,8 @@ PyErr_Display(PyObject *exception, PyObject *value, PyObject *tb)
 	int err = 0;
 	PyObject *f = PySys_GetObject("stderr");
 	Py_INCREF(value);
+	if (f == NULL)
+		_PyObject_Dump(value);
 	if (f == NULL)
 		fprintf(stderr, "lost sys.stderr\n");
 	else {