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/pystate.c

@@ -167,6 +167,8 @@ PyThreadState_New(PyInterpreterState *interp)
 
 		tstate->frame = NULL;
 		tstate->recursion_depth = 0;
+		tstate->overflowed = 0;
+		tstate->recursion_critical = 0;
 		tstate->tracing = 0;
 		tstate->use_tracing = 0;
 		tstate->tick_counter = 0;