Use smoltcp to replace our TCP code

[lf-]

When I was reading the smoltcp docs, I realized I had greatly misunderstood how extensible and well layered it is. We can probably throw away most of our TCP implementation in net_decode::tcp_reassemble if we use it. In particular, I think this is currently some of the worst code in the project, so it would be great to delete it.

https://docs.rs/smoltcp/latest/smoltcp/