Bring datum_image_eq() in line with datumIsEqual() by adding support for
comparing cstring datums.

An upcoming patch that adds deduplication to the nbtree AM will use
datum_image_eq().  datum_image_eq() will need to work with all datatypes
that can be used as the storage type of a B-Tree index column, including
cstring.  (cstring is used as the storage type for columns of type
"name" as a space-saving optimization.)

Discussion: https://postgr.es/m/CAH2-Wzn3Ee49Gmxb7V1VJ3-AC8fWn-Fr8pfWQebHe8rYRxt5OQ@mail.gmail.com