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.

Objects/moduleobject.c

@@ -22,7 +22,7 @@ PyModule_New(const char *name)
 	m = PyObject_GC_New(PyModuleObject, &PyModule_Type);
 	if (m == NULL)
 		return NULL;
-	nameobj = PyString_FromString(name);
+	nameobj = PyUnicode_FromString(name);
 	m->md_dict = PyDict_New();
 	if (m->md_dict == NULL || nameobj == NULL)
 		goto fail;