Improve list manipulation in several places

There was discussion in [1] about improvements to list manipulation in
several places.  But since the discussion is not related to the topic in
that thread, fork a new thread here and attach a patch to show my
thoughts.

Some are just cosmetic changes by using macros.  The others should have
performance gain from the avoidance of moving list entries.  But I doubt
the performance gain can be noticed or measured, as currently there are
only a few places affected by the change.  I still think the changes are
worthwhile though, because it is very likely that future usage of the
same scenario can benefit from these changes.

(Copying in David and Ranier.  Ranier provided a patch about the changes
in list.c, but I'm not using that one.)

[1]
https://www.postgresql.org/message-id/CAMbWs49aakL%3DPP7NcTajCtDyaVUE-NMVMGpaLEKreYbQknkQWA%40mail.gmail.com

Thanks
Richard