pgsql: Add json_strip_nulls and jsonb_strip_nulls functions.

Add json_strip_nulls and jsonb_strip_nulls functions.

The functions remove object fields, including in nested objects, that
have null as a value. In certain cases this can lead to considerably
smaller datums, with no loss of semantic information.

Andrew Dunstan, reviewed by Pavel Stehule.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/237a8824430c607fce1eafde0c625744d50a455c

Modified Files
--------------
doc/src/sgml/func.sgml | 23 ++++
src/backend/utils/adt/jsonfuncs.c | 197 +++++++++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 5 +
src/include/utils/json.h | 2 +
src/test/regress/expected/json.out | 50 +++++++++
src/test/regress/expected/json_1.out | 50 +++++++++
src/test/regress/expected/jsonb.out | 50 +++++++++
src/test/regress/expected/jsonb_1.out | 50 +++++++++
src/test/regress/sql/json.sql | 20 ++++
src/test/regress/sql/jsonb.sql | 19 ++++
11 files changed, 467 insertions(+), 1 deletion(-)