Add various missing conversions from and to Datum. The previous code
mostly relied on implicit conversions or its own explicit casts
instead of using the correct DatumGet*() or *GetDatum() functions.
We think these omissions are harmless. Some actual bugs that were
discovered during this process have been committed
separately (80c758a2e1, fd2ab03fea).
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8246d7ff-f4b7-4363-913e-827dadfeb145%40eisentraut.org
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