Implement IPv4 address configuration on Linux

This involved refactoring the crate structure to share code between
macOS and Linux. The new methods have not yet been implemented on
macOS, but they have todo!() placeholders.

tun/src/unix/queue.rs

@@ -0,0 +1,59 @@
+use fehler::throws;
+
+use std::{
+    io::{Error, Read, Write},
+    mem::MaybeUninit,
+    os::unix::io::{AsRawFd, IntoRawFd, RawFd},
+};
+
+use crate::TunInterface;
+
+pub struct TunQueue {
+    socket: socket2::Socket,
+}
+
+impl TunQueue {
+    #[throws]
+    pub fn recv(&self, buf: &mut [MaybeUninit<u8>]) -> usize {
+        self.socket.recv(buf)?
+    }
+}
+
+impl Read for TunQueue {
+    #[throws]
+    fn read(&mut self, buf: &mut [u8]) -> usize {
+        self.socket.read(buf)?
+    }
+}
+
+impl Write for TunQueue {
+    #[throws]
+    fn write(&mut self, buf: &[u8]) -> usize {
+        self.socket.write(buf)?
+    }
+
+    #[throws]
+    fn flush(&mut self) {
+        self.socket.flush()?
+    }
+}
+
+impl From<TunInterface> for TunQueue {
+    fn from(interface: TunInterface) -> TunQueue {
+        TunQueue {
+            socket: interface.socket,
+        }
+    }
+}
+
+impl AsRawFd for TunQueue {
+    fn as_raw_fd(&self) -> RawFd {
+        self.socket.as_raw_fd()
+    }
+}
+
+impl IntoRawFd for TunQueue {
+    fn into_raw_fd(self) -> RawFd {
+        self.socket.into_raw_fd()
+    }
+}