contrib: added ccan/json (BSD-MIT)

contrib/ccan/json/LICENSE

contrib/ccan/json/_info

contrib/ccan/json/json.c

contrib/ccan/json/json.h

contrib/ccan/json/test/common.h

contrib/ccan/json/test/run-construction.c

contrib/ccan/json/test/run-decode-encode.c

contrib/ccan/json/test/run-stringify.c

contrib/ccan/json/test/run-validate.c

contrib/ccan/json/test/test-strings

contrib/ccan/json/test/test-strings-reencoded

contrib/licenses/BSD-MIT