Marshal vs Serialize

[libertyeagle]

Just a remark here. Our send_fused only has data copying cost in sender end. On the receiver end, what we do is zero-cost deserialization, which fundamentally is still unmarshal, i.e., we reconstruct message in place.
We may also have more efficient serlization/fuse code by letting backend mrpc-derive generates a dedicated serialize and deserlize method, which we can potentially avoid sending irrelevant data, e.g., pointers like buf in Vec.