Suggestion: `derive(Len)` and `derive(IsEmpty)`

[George-Ogden]

Hi
Thanks for making this repo; it's full of great macros!
One thing I write a lot is this:

pub struct MyStruct(Vec<i32>) // or HashSet or BTreeMap

impl MyStruct {
    #[must_use]
    pub const fn is_empty(&self) -> bool {
        self.0.is_empty()
    }

    #[must_use]
    pub const fn len(&self) -> usize {
        self.0.len()
    }
    ...
}

I would like it a lot if I could derive this. And it's common enough (for me at least) that I would find a lot of value from it. The #[must_use] and const are just to satisfy clippy, and I think they aren't needed.

What do you think? How difficult would this be to implement?