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/Modules/gcmodule.c b/Modules/gcmodule.c
index 2dd058e..adcdb5f 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -713,7 +713,7 @@
 	double t1 = 0.0;
 
 	if (delstr == NULL) {
-		delstr = PyString_InternFromString("__del__");
+		delstr = PyUnicode_InternFromString("__del__");
 		if (delstr == NULL)
 			Py_FatalError("gc couldn't allocate \"__del__\"");
 	}