`control` functions in parsers should take the innermost type

In parser specs like:

#[derive(Parse)]
pub struct GeneveOverV6<Q: ByteSlice> {
    pub outer_eth: EthernetPacket<Q>,
    pub outer_v6: Ipv6Packet<Q>,
    #[ingot(from = "L4<Q>", control = my_control_a)]
    pub outer_udp: UdpPacket<Q>,
    pub outer_encap: GenevePacket<Q>,

    pub inner_eth: EthernetPacket<Q>,
    pub inner_l3: L3<Q>,
    pub inner_ulp: Ulp<Q>,
}

#[derive(Parse)]
pub struct NoEncap<Q: ByteSlice> {
    #[ingot(control = exit_on_arp)]
    pub inner_eth: EthernetPacket<Q>,
    #[ingot(control = my_control_b)]
    pub inner_l3: Option<L3<Q>>,
    pub inner_ulp: Option<Ulp<Q>>,
}

my_control_a operates on a &ValidL4<V>, whereas my_control_b operates on a &Option<ValidL3<V>>. It would make more sense if these were &ValidUdp<V> and &ValidL3<V> respectively -- in the former case because we have filtered all other protocols in the from tag, and in the latter because we can only be called when the layer is Some().

Start at the derive(Parse) implementation and the parser-spec handling for the control and from attributes; trace how the control argument type is selected for direct and Option fields. Done means control functions receive the innermost valid type, such as &ValidUdp<V> and &ValidL3<V>, for both examples, with coverage for these cases.