WIP: ICMP code/type firewall filters #759
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This is encoded using the same `Match` type as before, which captures ranges a bit more succintly. Also applies the same range matches to normal ports, which should greatly simplify huge ranges added in the control plane...
One of the side effects of wanting to filter on ICMP type is that flow IDs need to match that level of specificity. With the old FlowId design, we get *ICMP* and that's it, with the inner ID of an echo paylad added in occasionally. This can lead to fun cases like an allowed ICMP DU opening the door for *all* ICMP packets from that remote host. What we're doing now is providing access to the same two `u16`s in different contexts via `l4_info`/`_mut`, which lets us store the type/code in the source port field (cast as two `u8`s). This does need us to formalise some things we'd taken for granted, e.g., the `mirror` flow for an ICMP echo/request reply needs to also change out the expected type.
FelixMcFelix
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Still working out the right shape, types, etc.
Closes #730.