Support for `#[derive]` inside of nested Enums

[Crypto-Spartan]

Firstly, thank you for your work on this crate! It's very helpful & useful. 😃

Currently, the following will not compile with Nestify:

nest! {
    #[derive(Debug, Clone)]*
    pub(crate) struct Item {
        pub(crate) id: u16,
        pub(crate) category:
            #[derive(Copy, PartialEq, Eq)]*
            pub(crate) enum ItemCategory {
                Item(
                    #[derive(Deserialize)]    // expected non-macro attribute, found attribute macro `derive` not a non-macro attribute
                    pub(crate) enum ItemType {
                        Regular,
                        Starter,
                        Glyph,
                    }
                ),
                Relic,
                Consumable
            },
        pub(crate) name: Box<str>,
    }
}

But, if I change it to this, it does work (which, as far as I can tell, should be the same thing):

nest! {
    #[derive(Debug, Clone)]*
    pub(crate) struct Item {
        pub(crate) id: u16,
        pub(crate) category:
            #[derive(Copy, PartialEq, Eq)]*
            pub(crate) enum ItemCategory {
                ItemType(ItemType),
                Relic,
                Consumable
            },
        pub(crate) name: Box<str>,
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Deserialize)]
pub(crate) enum ItemType {
    Regular,
    Starter,
    Glyph,
}

This would work theoretically, but I have a custom deserialization already defined for ItemCategory, so it won't work for my use-case:

nest! {
    #[derive(Debug, Clone)]*
    pub(crate) struct Item {
        pub(crate) id: u16,
        pub(crate) category:
            #[derive(Copy, PartialEq, Eq, Deserialize)]*
            pub(crate) enum ItemCategory {
                Item(
                    pub(crate) enum ItemType {
                        Regular,
                        Starter,
                        Glyph,
                    }
                ),
                Relic,
                Consumable
            },
        pub(crate) name: Box<str>,
    }
}

I'm not sure if this is a bug or intended behavior, but it would be really great if it was possible to still use #[derive(Deserialize)] on an Enum inside of an Enum.

[snowfoxsh]

Thank you! I will look into this