Use stack allocated fixed-size vectors where possible

[dtzxporter]

Since processing of packets is likely to happen thousands of times per second, there will be a performance gain if the struct doesn't allocate to Vec<> every time you unmarshal a header.

Something like arrayvec would be suitable for this: https://crates.io/crates/arrayvec

[k0nserv]

I do agree about reducing the number of heap allocations, but in a future context I think arrayvec is not advisable because it makes the futures themselves too large. Further, I think it complicates achieving cancel safety since the stack gets dropped when a future is cancelled.

The cancel safety point is equally true for naive heap allocations of course.