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/Lib/test/test_new.py b/Lib/test/test_new.py
index c919621..797a8c3 100644
--- a/Lib/test/test_new.py
+++ b/Lib/test/test_new.py
@@ -143,7 +143,7 @@
                 firstlineno, lnotab)
 
             # new.code used to be a way to mutate a tuple...
-            class S(str8):
+            class S(str):
                 pass
             t = (S("ab"),)
             d = new.code(argcount, kwonlyargcount, nlocals, stacksize,