feat: Improve performance of Exchange

[oblique]

We sacrifice memory usage for performance.

[zvolin]

cool 🔥

[zvolin]

Miri has no issues with it but I'm concerned about the transmute.

In the MaybeUninit docs there is this example (3rd in section)
https://doc.rust-lang.org/std/mem/union.MaybeUninit.html#initialization-invariant

let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️

so I think it would also do so for

let x: MaybeUninit<u8> = MaybeUninit::uninit();
let x: u8 = std::mem::transmute(x); // ub

also I found this MaybeUninit::slice_assume_init_mut which happens to do the same thing and it says that calling is unsafe on uninitialized data
https://doc.rust-lang.org/src/core/mem/maybe_uninit.rs.html#976

Want to open a discussion what do we want to do with it. It seems it's safe by current compiler's behavior and used in some popular crates but causes UB per documentation.

[oblique]

I posted a question about this in the Rust forum (link) and the code is safe as long as Stream only fills the slice. If Stream::poll_read try to read a byte from the slice that wasn't filled yet, then it is UB.

I have checked all the Stream types we use (tokio-tcp, noise, yamux, quic) and they only fill the slice. But for the sake of peace of mind for future releases of the mentioned types, I'm going to remove the unsafe. By removing a ~1ms of overhead will be added.

[zvolin]

Nice

[fl0rek]

great work! 🚀