Fix clippy::from_over_into lint

[taiki-e]

warning: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
   --> src/tcp.rs:231:1
    |
231 | impl Into<Arc<Async<std::net::TcpListener>>> for TcpListener {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(clippy::from_over_into)]` on by default
    = help: consider to implement `From` instead
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#from_over_into