Packing for nested structs wrong when using partial integer types

[paulzhng]

Hi,

I noticed that packing nested structs fails for partial Integer types when used in inner structs. Weirdly enough, this is only applicable for bits smaller than 5.

Following fields packed result in [0] being outputted.

use packed_struct::prelude::*;

#[derive(PackedStruct, PartialEq, Debug)]
#[packed_struct(bit_numbering = "msb0", endian = "msb")]
pub struct Outer {
    #[packed_field(bits = "0")]
    inner_field: Inner,
}

#[derive(PackedStruct, PartialEq, Debug)]
#[packed_struct(bit_numbering = "msb0", endian = "msb")]
pub struct Inner {
    #[packed_field(bits = "0")]
    val: Integer<u8, packed_bits::Bits<1>>,
}

fn main() {
    let outer = Outer {
        inner_field: Inner { val: 1.into() },
    };
    let packed = outer.pack().unwrap();
    let unpacked = Outer::unpack_from_slice(&packed).unwrap();

    println!("unpacked: {:#}", unpacked);
    println!("datetime: {:#}", outer);
    assert_eq!(outer, unpacked);
}

Code example also available here: rustexplorer.com.