Provide a `TcpSocket` abstraction

[piegamesde]

I'd like to have a TcpSocket implementation like in Tokio (I don't really want to depend on the Tokio runtime just for this feature). I know that there are workarounds, but an async-first, easy to use implementation would be really appreciated.

[taiki-e]

Since async-net already depends on socket2 via async-io, this could probably be implemented in a way that wraps socket2. (Most of the work probably needs to be done on the async-net side to avoid making socket2 a public dependency.)