Add support for seqpacket sockets

[kanpov]

Analogous to #48 , but for SOCK_SEQPACKET.

https://groups.oasis-open.org/higherlogic/ws/public/ballot?id=3680
https://lore.kernel.org/netdev/20210421093554.y45al5r7xhoo7dwh@steredhat/T/
https://docs.oasis-open.org/virtio/virtio/v1.3/virtio-v1.3.html

Both this and #48 necessitate support in tokio-vsock as well, I'd suggest a major version bump with VsockStream renamed to VsockStreamSocket, with VsockDatagramSocket and VsockSeqpacketSocket being introduced.