style: make poll_read_exact a general trait

[VendettaReborn]

re-open of #675

🤔 This is a ...

• Code style optimization

💡 Background and solution

there exists some similar code in vmess and shadowtls stream, which i found when i re-read the code.

the poll_read_exact method is especially useful for poll based packet decoding, many proxy projects have their own implement, but there are some ways to make it a general method with a base trait to provide the (inner stream, BytesMut, read_pos).
so here comes the base trait ReadExactBase, and ReadExact trait is automatically implemented for all types which have implemented ReadExactBase

pub trait ReadExactBase {
    /// inner stream to be polled
    type I: AsyncRead + Unpin;
    /// prepare the inner stream, read buffer and read position
    fn decompose(&mut self) -> (&mut Self::I, &mut BytesMut, &mut usize);
}

pub trait ReadExt: ReadExactBase {
    fn poll_read_exact(
        &mut self,
        cx: &mut std::task::Context,
        size: usize,
    ) -> Poll<std::io::Result<()>>;
}

[ibigbug]

nice!