nom::length_bytes

macro_rules! length_bytes {
    ($i:expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($i:expr, $f:expr) => { ... };
}

`length_bytes!(&[T] -> IResult<&[T], nb>) => &[T] -> IResult<&[T], &[T]> gets a number from the first parser, then extracts that many bytes from the remaining stream