Firezone's DNS resource NAT can only route certain ICMP packets. In
particular, we can only route ICMP _echo_ packets as only those have a
stable identifier that we can use in our NAT table.
To clarify that, rename the enum variant to `IcmpEcho` from `Icmp`.
In a recent refactor, we appear to have broken DNS resources that are
IPv6 only. This was caused by a change in how we iterate over the proxy
IP mappings. By bailing out as soon as we "run out" of IP mappings, we
actually never get to the IPv6 mappings.
This PR fixes this behaviour and adds a regression test to ensure we
always insert an entry for all proxy IPs.
The current Rust workspace isn't as consistent as it could be. To make
navigation a bit easier, we move a few crates around. Generally, we
follow the idea that entry-points should be at the top-level. `rust/`
now looks like this (directories only):
```
.
├── cli # Firezone CLI
├── client-ffi # Entry point for Apple & Android
├── gateway # Gateway
├── gui-client # GUI client
├── headless-client # Headless client
├── libs # Library crates
├── relay # Relay
├── target # Compile artifacts
├── tests # Crates for testing
└── tools # Local tools
```
To further enforce this structure, we also drop the `firezone-` prefix
from all crates that are not top-level binary crates.
