jsonapi: Use const char *
Apply const qualifiers to char * arguments and fields throughout the
jsonapi. This allows the top-level APIs such as
pg_parse_json_incremental() to declare their input argument as const.
It also reduces the number of unconstify() calls.
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://www.postgresql.org/message-id/flat/
f732b014-f614-4600-a437-
dba5a2c3738b%40eisentraut.org