feat: support customized offset

[MrCroxx]

Support customized offset for indirect wrapper usage:

#[cfg(test)]
mod tests {
    use crate::LinkedListLink;
    use core::cell::UnsafeCell;
    use std::rc::Rc;

    struct Obj {
        link: LinkedListLink,
    }

    struct Wrapper {
        obj: Obj,
    }

    struct IndirectWrapper {
        cell: UnsafeCell<Obj>,
    }

    intrusive_adapter! {
        /// Test doc comment
        ObjAdapter1 = Rc<Obj>: Obj { link => LinkedListLink }
    }

    intrusive_adapter! {
        /// Test doc comment
        WrapperAdapter1 = Rc<Wrapper>: Wrapper { obj.link => LinkedListLink }
    }

    intrusive_adapter! {
        /// Test doc comment
        IndirectWrapperAdapter1 = Rc<Wrapper>: Wrapper { ?offset = crate::offset_of!(IndirectWrapper, cell) + crate::offset_of!(Obj, link) => LinkedListLink }
    }
}

Signed-off-by: MrCroxx [email protected]Signed-off-by: MrCroxx [email protected]