freeze Encoding objects

Encoding objects can be accessed in multi-ractors and there is no
state to mutate. So we can mark it as frozen and shareable.
[Bug #17188]

encoding.c

@@ -122,7 +122,10 @@ rb_data_is_encoding(VALUE obj)
 static VALUE
 enc_new(rb_encoding *encoding)
 {
-    return TypedData_Wrap_Struct(rb_cEncoding, &encoding_data_type, (void *)encoding);
+    VALUE enc = TypedData_Wrap_Struct(rb_cEncoding, &encoding_data_type, (void *)encoding);
+    rb_obj_freeze(enc);
+    FL_SET_RAW(enc, RUBY_FL_SHAREABLE);
+    return enc;
 }
 
 static void