parse_ident: Add const to two parameters

This function doesn't change anything in the string delimitted by these
parameters, and future commits will call it with const strings that
otherwise would have to cast away const

Author: khwilliamson

embed.fnc

@@ -6176,8 +6176,8 @@ So	|SV *	|new_constant	|NULLOK const char *s			\
 				|NULLOK const char *type		\
 				|STRLEN typelen 			\
 				|NULLOK const char **error_msg
-S	|char * |parse_ident	|SPTR char *s				\
-				|EPTRQ char * const s_end		\
+S	|char * |parse_ident	|SPTR const char *s			\
+				|EPTRQ const char * const s_end 	\
 				|SPTR char **d				\
 				|EPTR char * const e			\
 				|bool is_utf8				\

proto.h

@@ -10539,7 +10539,7 @@ S_new_constant(pTHX_ const char *s, STRLEN len, const char *key, STRLEN keylen,
 }
 
 STATIC char *
-S_parse_ident(pTHX_ char *s, char * const s_end,
+S_parse_ident(pTHX_ const char *s, const char * const s_end,
                     char **d, char * const e,
                     bool is_utf8, U32 flags)
 {
@@ -10591,7 +10591,7 @@ S_parse_ident(pTHX_ char *s, char * const s_end,
 
             /* Find the end of the identifier by accumulating characters until
              * find a non-identifier character */
-            char *t = s + advance;
+            const char *t = s + advance;
             while ((advance = isIDCONT_utf8_safe((const U8*) t,
                                                  (const U8*) s_end)))
             {
@@ -10638,7 +10638,11 @@ S_parse_ident(pTHX_ char *s, char * const s_end,
                    identifier*/
             break;
     }
-    return s;
+
+    /* Cast away const, because many of our callers don't have it; this
+     * function declares it as const so as to indicate that it doesn't change
+     * it, and it can be called using a const parameter */
+    return (char *) s;
 }
 
 char *