Add CR about packed structs

Author: Ekdohibs

ocaml/runtime/caml/mlvalues.h

@@ -249,6 +249,7 @@ CAMLextern value caml_get_public_method (value obj, value tag);
    The remaining bits are the field number for the first word of the
    environment, or, in other words, the offset (in words) from the closure
    to the environment part. */
+/* CR ncourant: it might be cleaner to use a packed struct here */
 #ifdef ARCH_SIXTYFOUR
 #define Arity_closinfo(info) ((intnat)(info) >> 56)
 #define Start_env_closinfo(info) (((uintnat)(info) << 9) >> 10)