Ingress socket data through borrowed buffer

[MathiasKoch]

Currently the socket data ingress works as

atat::Buf -> serde_json::parse -> SocketData -> hex::decode -> Tcp::RxBuf
          ^                    ^             ^              ^
        clone                clone         inline         clone

Using serde's zero-copy deserializing (https://serde.rs/lifetimes.html), it should be possible to refactor this into:

atat::Buf -> serde_json::parse -> SocketData -> Tcp::RxBuf
          ^                    ^             ^
        clone                borrow        borrow

Where the two borrow's symbolise the borrowed Tcp::RxBuf

This feature is not currently supported by neither serde_at, nor atat_derive, and might not need to be supported by atat_derive at all.