deduplicate static rb_str_format format strings

Anybody who hits these code paths can hit them again in the
future, so try deduplicating across multiple runs of these
methods to reduce garbage.

* string.c (str_upto_each): fstring on "%.*d"
* strftime.c (rb_strftime_with_timespec): fstring on "%0*d"

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57997 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

strftime.c

@@ -820,7 +820,8 @@ rb_strftime_with_timespec(VALUE ftime, const char *format, size_t format_len,
                                         VALUE args[2], result;
                                         args[0] = INT2FIX(precision);
                                         args[1] = subsec;
-                                        result = rb_str_format(2, args, rb_str_new2("%0*d"));
+                                        result = rb_str_format(2, args,
+                                                      rb_fstring_cstr("%0*d"));
                                         (void)strlcpy(s, StringValueCStr(result), endp-s);
                                         s += precision;
                                 }

string.c

@@ -4147,7 +4147,7 @@ str_upto_each(VALUE beg, VALUE end, int excl, int (*each)(VALUE, VALUE), VALUE a
 	}
 	else {
 	    ID op = excl ? '<' : idLE;
-	    VALUE args[2], fmt = rb_obj_freeze(rb_usascii_str_new_cstr("%.*d"));
+	    VALUE args[2], fmt = rb_fstring_cstr("%.*d");
 
 	    args[0] = INT2FIX(width);
 	    while (rb_funcall(b, op, 1, e)) {