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/Include/pystate.h b/Include/pystate.h
index 4919d99..0681e65 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -61,6 +61,10 @@
 
     struct _frame *frame;
     int recursion_depth;
+    char overflowed; /* The stack has overflowed. Allow 50 more calls
+		        to handle the runtime error. */
+    char recursion_critical; /* The current calls must not cause 
+				a stack overflow. */
     /* 'tracing' keeps track of the execution depth when tracing/profiling.
        This is to prevent the actual trace/profile code from being recorded in
        the trace/profile. */