Raw ICMP Socket Responds to Echo Requests Automatically

When using a raw ICMP socket, it will automatically respond to echo requests. Typically when users create raw sockets, they want full control over the behavior, and only want to use the socket as a means of ingressing/egressing packets. Is this an intended feature of the `smoltcp` library? I'm happy to open a pull request to address this if this is not an intended feature.