`CommandBuilder::serialize_into` does not return length of data

[dishmaker]

Problem

Currently, to get the resulting buffer after encoding CommandBuilder, length needs to be calculated separately.

let len = builder.required_len();
builder.serialize_into(&mut &mut buf[..]).unwrap();
&buf[..len]

Solution

It would be nice if serialize_into returned length too:

let len = builder.serialize_into(&mut &mut buf[..]).unwrap();
&buf[..len]

Proposal

Even better - let's encode directly to slice like der::Encode:
https://docs.rs/der/0.8.0-rc.10/der/trait.Encode.html#method.encode_to_slice

fn encode_to_slice<'a>(&self, buf: &'a mut [u8]) -> Result<&'a [u8]>

[Spoiler] Encoding with iso7816 0.2.0

use iso7816::command::{CommandBuilder, ExpectedLen, class::Class};

fn main() {
    let mut buf = [0u8; 16];
    let read_binary = {
        let builder = CommandBuilder::<()>::new(
            Class::from_byte(0x00).unwrap(),
            iso7816::Instruction::ReadBinary,
            0x00,
            0x00,
            (),
            ExpectedLen::Ne(65535),
        )
        .force_extended();
        let len = builder.required_len();
        builder.serialize_into(&mut &mut buf[..]).unwrap();
        &buf[..len]
    };

    println!("read binary (extended) C-APDU: {:?}", read_binary);
}