dir.c, io.c: use rb_id_encoding

* dir.c (dir_initialize): rb_id_encoding() returns same ID with
  caching.

* io.c (Init_IO): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53012 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

dir.c

@@ -491,7 +491,7 @@ dir_initialize(int argc, VALUE *argv, VALUE dir)
     const char *path;
 
     if (!keyword_ids[0]) {
-	keyword_ids[0] = rb_intern("encoding");
+	keyword_ids[0] = rb_id_encoding();
     }
 
     fsenc = rb_filesystem_encoding();

io.c

@@ -12494,7 +12494,7 @@ Init_IO(void)
     sym_flags = ID2SYM(rb_intern("flags"));
     sym_extenc = ID2SYM(rb_intern("external_encoding"));
     sym_intenc = ID2SYM(rb_intern("internal_encoding"));
-    sym_encoding = ID2SYM(rb_intern("encoding"));
+    sym_encoding = ID2SYM(rb_id_encoding());
     sym_open_args = ID2SYM(rb_intern("open_args"));
     sym_textmode = ID2SYM(rb_intern("textmode"));
     sym_binmode = ID2SYM(rb_intern("binmode"));