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.

Python/future.c

@@ -55,7 +55,7 @@ future_parse(PyFutureFeatures *ff, mod_ty mod, const char *filename)
 
 	static PyObject *future;
 	if (!future) {
-		future = PyString_InternFromString("__future__");
+		future = PyUnicode_InternFromString("__future__");
 		if (!future)
 			return 0;
 	}