Support skipping only some variants with ZeroizeOnDrop

[daxpedda]

I noticed that the following would fail to compile:

#[derive_where(ZeroizeOnDrop)]
enum Test<T> {
    A(PhantomData<T>),
    #[derive_where(skip_inner)]
    B(PhantomData<T>),
}

Because it produces the following:

impl<T> Drop for Test<T> {
    fn drop(&mut self) {
        match self {
            Test::A(ref mut __field_0) => {
                __field_0.zeroize_or_on_drop();
            }
        }
    }
}

Instead, we now make sure to also cover variants that are skipped, but their body is just empty now.

See the test I added for a more complete example.

[daxpedda]

I discovered another bug!

Apparently the following:

#[derive_where(ZeroizeOnDrop)]
enum Test<T> {
    A(PhantomData<T>),
    #[derive_where(skip_inner)]
    B(PhantomData<T>),
}

Would generate multiple calls to Zeroize::zeroize():

impl<T> Drop for Test<T> {
    fn drop(&mut self) {
        Zeroize::zeroize(self);
        Zeroize::zeroize(self);
    }
}