Add const to values and nulls arguments

There are a lot of Datum *values, bool *nulls argument pairs that should 
really be const.  The 0001 patch makes those changes.  Some of these 
hunks depend on each other.

The 0002 patch, which I'm not proposing to commit at this time, makes 
similar changes but in a way that breaks the table and index AM APIs. 
So I'm just including that here in case anyone wonders, why didn't you 
touch those.  And also maybe if we ever change that API incompatibly we 
could throw this one in then.