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.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 84b3384..6e63852 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -207,7 +207,7 @@
 		null_error();
 		return -1;
 	}
-	okey = PyString_FromString(key);
+	okey = PyUnicode_FromString(key);
 	if (okey == NULL)
 		return -1;
 	ret = PyObject_DelItem(o, okey);
@@ -1598,7 +1598,7 @@
 	if (key == NULL)
 		return null_error();
 
-	okey = PyString_FromString(key);
+	okey = PyUnicode_FromString(key);
 	if (okey == NULL)
 		return NULL;
 	r = PyObject_GetItem(o, okey);
@@ -1617,7 +1617,7 @@
 		return -1;
 	}
 
-	okey = PyString_FromString(key);
+	okey = PyUnicode_FromString(key);
 	if (okey == NULL)
 		return -1;
 	r = PyObject_SetItem(o, okey, value);
@@ -1989,11 +1989,13 @@
 	PyObject *bases;
 
 	if (__bases__ == NULL) {
-		__bases__ = PyString_FromString("__bases__");
+		__bases__ = PyUnicode_FromString("__bases__");
 		if (__bases__ == NULL)
 			return NULL;
 	}
+	Py_ALLOW_RECURSION
 	bases = PyObject_GetAttr(cls, __bases__);
+	Py_END_ALLOW_RECURSION
 	if (bases == NULL) {
 		if (PyErr_ExceptionMatches(PyExc_AttributeError))
 			PyErr_Clear();
@@ -2067,7 +2069,7 @@
 	int retval = 0;
 
 	if (__class__ == NULL) {
-		__class__ = PyString_FromString("__class__");
+		__class__ = PyUnicode_FromString("__class__");
 		if (__class__ == NULL)
 			return -1;
 	}