pgsql: Split JSON lexer/parser from 'json' data type support.

Split JSON lexer/parser from 'json' data type support.

Keep the code that pertains to the 'json' data type in json.c, but
move the lexing and parsing code to a new file jsonapi.c, a name
I chose because the corresponding prototypes are in jsonapi.h.

This seems like a logical division, because the JSON lexer and parser
are also used by the 'jsonb' data type, but the SQL-callable functions
in json.c are a separate thing. Also, the new jsonapi.c file needs to
include far fewer header files than json.c, which seems like a good
sign that this is an appropriate place to insert an abstraction
boundary. I took the opportunity to remove a few apparently-unneeded
includes from json.c at the same time.

Patch by me, reviewed by David Steele, Mark Dilger, and Andrew
Dunstan. The previous commit was, too, but I forgot to note it
in the commit message.

Discussion: http://postgr.es/m/CA+TgmoYfOXhd27MUDGioVh6QtpD0C1K-f6ObSA10AWiHBAL5bA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/11b5e3e35d3900164cb36754ee4e4dcab0bd02f0

Modified Files
--------------
src/backend/utils/adt/Makefile  |    1 +
src/backend/utils/adt/json.c    | 1206 +-------------------------------------
src/backend/utils/adt/jsonapi.c | 1216 +++++++++++++++++++++++++++++++++++++++
src/include/utils/jsonapi.h     |    6 +
4 files changed, 1224 insertions(+), 1205 deletions(-)