feat: `decode_in_slice()`

[jarkkojs]

For memory constrained enviroments and network protocols being able to use memory without extra copies is a benefit. E.g. in my ZMODEM2 crate I need to do this for hex messages:

if encoding == Encoding::ZHEX {
    let mut tmp = [0; (FrameHeader::unescaped_size(Encoding::ZHEX) - 1) / 2];
    match hex::decode_to_slice(out, &mut tmp) {
        Ok(x) => x,
        _ => {
            log::error!("from_hex error");
            return Err(ErrorKind::InvalidData.into());
        }
    }
    out.clear();
    out.extend_from_slice(&tmp);
}

It is good example since the protocol frames can come also in binary format in addition to hex. It would be nice to do this preprocessing within the same buffer as where the data landed to make a transparent zerocopy flow.

Link: https://github.com/jarkkojs/zmodem2/

[jarkkojs]

Asymmetry is actually cool here. There's much less use for encode counter-part as you don't end up doing extra copies with current functions, as you build your buffer from zero. So I'd consider this without encode_in_slice. It is also less nice in the sense that it needs more space than the original.

[jarkkojs]

I adapted it to my zmodem2 crate:

        if encoding == Encoding::ZHEX {
            hex::decode_in_slice(&mut out).or::<io::Error>(Err(ErrorKind::InvalidData.into()))?;
            out.truncate(out.len() / 2);
        }

I'd like to make this scale from microcontroller to xeon so thus I'm proactively interested on saving in this case 7 byte of stack space.

Relative cost of the copy is +50%. For the record, I use tinyvec by Google for the stack based vector (just for the context of the example).