Update `write::BufWriter` to use uninitialized slice

[NobodyXu]

async-compression/crates/async-compression/src/futures/write/buf_writer.rs

Line 22 in 386f59e

buf: Box<[u8]>,

[NobodyXu]

Also, it's possible to remove the copy_within call completely, since we can just use poll_write_vectored call instead.

While it is usually implemented using poll_write, it is still much better than copy_within which translates to memmove, it gets especially bad when each flush only writes a few data.

Plus there are writer that actually implements poll_write_vectored.

And with that change, poll_write could write its internal buffer and extra buffer at the same time.

[NobodyXu]

For poll_write impl, we should try to flush internal buffer + write new data in one go, if says, half of the buffer is written with unflushed data

[NobodyXu]

Though for poll_..._buffer we still want to get a whole buffer as long as possible...seems like we are recreating bytes::BytesMut

Edit:

We should probably have a rule based on the size, if it's smaller than a few hundred bytes then definitely try flushing first before encoding/decoding new data