pgsql: Consolidate ItemPointer to Datum conversion functions

Consolidate ItemPointer to Datum conversion functions

Instead of defining the same set of macros several times, define it
once in an appropriate header file.  In passing, convert to inline
functions.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/844dd4c5-e5a1-3df1-bfaf-d1e1c2a16e45%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bd944884e92af7082063428a1124a15041597c33

Modified Files
--------------
contrib/pageinspect/btreefuncs.c |  2 --
contrib/pageinspect/ginfuncs.c   |  3 ---
contrib/pageinspect/gistfuncs.c  |  2 --
src/backend/utils/adt/tid.c      |  5 -----
src/include/storage/itemptr.h    | 20 ++++++++++++++++++++
5 files changed, 20 insertions(+), 12 deletions(-)