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/Parser/tokenizer.c b/Parser/tokenizer.c
index f3eeb2c..e7dada6 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -18,6 +18,17 @@
 #include "abstract.h"
 #endif /* PGEN */
 
+#define is_potential_identifier_start(c) (\
+                          (c >= 'a' && c <= 'z')\
+		       || (c >= 'A' && c <= 'Z')\
+		       || c == '_')
+
+#define is_potential_identifier_char(c) (\
+                          (c >= 'a' && c <= 'z')\
+		       || (c >= 'A' && c <= 'Z')\
+		       || (c >= '0' && c <= '9')\
+		       || c == '_')
+
 extern char *PyOS_Readline(FILE *, FILE *, char *);
 /* Return malloc'ed string including trailing \n;
    empty malloc'ed string for EOF;
@@ -1209,7 +1220,7 @@
 	}
 
 	/* Identifier (most frequent token!) */
-	if (isalpha(c) || c == '_') {
+	if (is_potential_identifier_start(c)) {
 		/* Process r"", u"" and ur"" */
 		switch (c) {
 		case 'r':
@@ -1227,7 +1238,7 @@
 				goto letter_quote;
 			break;
 		}
-		while (isalnum(c) || c == '_') {
+		while (is_potential_identifier_char(c)) {
 			c = tok_nextc(tok);
 		}
 		tok_backup(tok, c);