Use zend_string for arg_separators

This allows us to avoid a call to `zend_ini_str` which took 6% of the
profile on my i7-4790 for a call to `http_build_query`. Now we can just
grab the value from the globals.
In other files this can avoid some length recomputations.

main/php_variables.c

@@ -527,7 +527,7 @@ SAPI_API SAPI_TREAT_DATA_FUNC(php_default_treat_data)
 	switch (arg) {
 		case PARSE_GET:
 		case PARSE_STRING:
-			separator = PG(arg_separator).input;
+			separator = ZSTR_VAL(PG(arg_separator).input);
 			break;
 		case PARSE_COOKIE:
 			separator = ";\0";