Index tuple data arrays using Anum_xxx symbolic constants instead of "i++".

We had already converted most places to this style, but this patch gets the
last few that were still doing it the old way.  The main advantage is that
this exposes a greppable name for each target column, rather than having
to rely on comments (which a couple of places failed to provide anyhow).

Richard Hopkins, additional work by me to clean up update_attstats() too