Add Read and Write for Async TunInterface

Those features are implemented using AsyncFD. While write doesn't
require a mutable reference to self, read does.

Make Async Tun a feature

remove async tun from workspace

rename write/read to send/recv

tun/src/lib.rs

@@ -8,5 +8,9 @@ pub(crate) mod imp;
 
 mod options;
 
+#[cfg(any(target_os = "linux", target_vendor = "apple"))]
+#[cfg(feature = "tokio")]
+pub mod tokio;
+
 pub use imp::{TunInterface, TunQueue};
 pub use options::TunOptions;