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/funcobject.c b/Objects/funcobject.c
index 6f17e7a..ff1b4c8 100644
--- a/Objects/funcobject.c
+++ b/Objects/funcobject.c
@@ -322,7 +322,7 @@
 
 	/* Not legal to del f.func_name or to set it to anything
 	 * other than a string object. */
-	if (value == NULL || !PyString_Check(value)) {
+	if (value == NULL || (!PyString_Check(value) && !PyUnicode_Check(value))) {
 		PyErr_SetString(PyExc_TypeError,
 				"__name__ must be set to a string object");
 		return -1;
@@ -516,7 +516,7 @@
 	if (nfree != nclosure)
 		return PyErr_Format(PyExc_ValueError,
 				    "%s requires closure of length %zd, not %zd",
-				    PyString_AS_STRING(code->co_name),
+				    PyUnicode_AsString(code->co_name),
 				    nfree, nclosure);
 	if (nclosure) {
 		Py_ssize_t i;