Convert *GetDatum() and DatumGet*() macros to inline functions

Convert *GetDatum() and DatumGet*() macros to inline functions

The previous macro implementations just cast the argument to a target
type but did not check whether the input type was appropriate.  The
function implementation can do better type checking of the input type.

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/8528fb7e-0aa2-6b54-85fb-0c0886dbd6ed%40enterprisedb.com