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.

Modules/_codecsmodule.c

@@ -172,7 +172,7 @@ escape_encode(PyObject *self,
 			      &PyString_Type, &str, &errors))
 		return NULL;
 
-	size = PyUnicode_GET_SIZE(str);
+	size = PyString_GET_SIZE(str);
 	newsize = 4*size;
 	if (newsize > PY_SSIZE_T_MAX || newsize / 4 != size) {
 		PyErr_SetString(PyExc_OverflowError,