json: fix escaping of object keys

json: fix escaping of object keys

gcc/ChangeLog:
	* json.cc (print_escaped_json_string): New, taken from
	string::print.
	(object::print): Use it for printing keys.
	(string::print): Move implementation to
	print_escaped_json_string.
	(selftest::test_writing_objects): Add a key containing
	quote, backslash, and control characters.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>