XRBK macro: allow deriving ReadableWithContext by placing a #[read_with(...)] attribute on the item

[Antikyth]

Examples

#[derive(X11Size, ReadableWithContext)]
#[read_with(len: &usize)]
pub struct MyVec<T> {
    #[context(len => *len)]
    vec: Vec<T>,
}

Which could derive the following:

#[automatically_derived]
impl<T: Readable + X11Size> ReadableWithContext for MyVec<T> {
    type Context = (usize);

    fn read_with(buf: &mut impl Buf, (param_len): &(usize)) -> Result<Self, ReadError> {
        fn field_vec(len: &usize) -> <Vec<T>>::Context {
            *len
        }

        let field_vec = <Vec<T>>::read_with(buf, field_vec(param_len));

        Ok(Self {
            vec: field_vec,
        })
    }
}