Usage of designated initializers came up in:
https://www.postgresql.org/message-id/flat/ZdWXhAt9Tz4d-lut%40paquier.xyz#9dc17e604e58569ad35643672bf74acc
This converts all arrays that I could find that could clearly benefit
from this without any other code changes being necessary.
There were a few arrays that I didn't convert that seemed like they
could be useful to convert, but where the variables started counting
at 1. So by converting them elements the array would grow and elements
would be shifted by one. Changing those might be nice, but would
require some more code changes so I didn't want to combine it with
these simpler refactors. The arrays I'm talking about were
specifically tsearch_op_priority, BT_implies_table, BT_refutes_table,
and BT_implic_table.
