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.
