[for PG16]
There are many calls to construct_array() and deconstruct_array() for
built-in types, for example, when dealing with system catalog columns.
These all hardcode the type attributes necessary to pass to these functions.
To simplify this a bit, add construct_array_builtin(),
deconstruct_array_builtin() as wrappers that centralize this hardcoded
knowledge. This simplifies many call sites and reduces the amount of
hardcoded stuff that is spread around.
I also considered having genbki.pl generate lookup tables for these
hardcoded values, similar to schemapg.h, but that ultimately seemed
excessive.
Thoughts?