arcanist: update

Signed-off-by: Austin Seipp <aseipp@pobox.com>
compcert: 2.2 -> 2.3pl2

Signed-off-by: Austin Seipp <aseipp@pobox.com>
dd-agent: 4.2.1 -> 4.3.1

Signed-off-by: Austin Seipp <aseipp@pobox.com>