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/frameobject.c

@@ -542,7 +542,7 @@ static PyObject *builtin_object;
 
 int _PyFrame_Init()
 {
-	builtin_object = PyString_InternFromString("__builtins__");
+	builtin_object = PyUnicode_InternFromString("__builtins__");
 	return (builtin_object != NULL);
 }
 
@@ -722,7 +722,7 @@ map_to_dict(PyObject *map, Py_ssize_t nmap, PyObject *dict, PyObject **values,
 	for (j = nmap; --j >= 0; ) {
 		PyObject *key = PyTuple_GET_ITEM(map, j);
 		PyObject *value = values[j];
-		assert(PyString_Check(key));
+		assert(PyString_Check(key)/*XXX this should go*/ || PyUnicode_Check(key));
 		if (deref) {
 			assert(PyCell_Check(value));
 			value = PyCell_GET(value);
@@ -770,7 +770,7 @@ dict_to_map(PyObject *map, Py_ssize_t nmap, PyObject *dict, PyObject **values,
 	for (j = nmap; --j >= 0; ) {
 		PyObject *key = PyTuple_GET_ITEM(map, j);
 		PyObject *value = PyObject_GetItem(dict, key);
-		assert(PyString_Check(key));
+		assert(PyUnicode_Check(key));
 		/* We only care about NULLs if clear is true. */
 		if (value == NULL) {
 			PyErr_Clear();