Add missing Datum conversions

Add missing Datum conversions

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 (80c758a2e1d, fd2ab03fea2).

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8246d7ff-f4b7-4363-913e-827dadfeb145%40eisentraut.org