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

.vscode/settings.json

@@ -19,5 +19,5 @@
     ],
     "[rust]": {
         "editor.defaultFormatter": "rust-lang.rust-analyzer",
-    },
+    }
 }