Fix a parser bug.

Fix a bug where we were incorrectly allowing trailing commas in JSON objects,
for example:

    { "foo" : null, }

Add predicates that perform input folds over JSON object members.

src/json.json_parser.m:
	Fix the above bug.

src/json.m:
	Add predicates for folding over the members of a JSON object.
	Each member is only read after the accumulator predicate has
	been applied to the previous one.

tests/trailing_object_comma.{json,exp}:
	Regression test for the above.