pgsql: Expose more out/readfuncs support functions.

Expose more out/readfuncs support functions.

Previously bcac23d exposed a subset of support functions, namely the
ones Kaigai found useful. In
20160304193704.elq773pyg5fyl3mi@alap3.anarazel.de I mentioned that
there's some functions missing to use the facility in an external
project.

To avoid having to add functions piecemeal, add all the functions which
are used to define READ_* and WRITE_* macros; users of the extensible
node functionality are likely to need these. Additionally expose
outDatum(), which doesn't have it's own WRITE_ macro, as it needs
information from the embedding struct.

Discussion: 20160304193704.elq773pyg5fyl3mi@alap3.anarazel.de

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c1ddd2361f6eb071d51b856c697a4aab22f8c776

Modified Files
--------------
src/backend/nodes/outfuncs.c  | 22 ++++++++++------------
src/backend/nodes/readfuncs.c | 16 +++++-----------
src/include/nodes/nodes.h     |  8 ++++++++
3 files changed, 23 insertions(+), 23 deletions(-)