Support module level attribute

[mversic]

this can be supported if there is need or interest:

use core::mem::ManuallyDrop;

pub trait Dispatch {
    type Group;
}

pub enum GroupA {}
impl Dispatch for String {
    type Group = GroupA;
}
impl<T> Dispatch for Vec<T> {
    type Group = GroupA;
}

pub enum GroupB {}
impl Dispatch for i32 {
    type Group = GroupB;
}
impl Dispatch for u32 {
    type Group = GroupB;
}

pub enum GroupC {}
pub enum GroupD {}

#[disjoint_impls::disjoint_impls_mod]
pub mod kita {
    use super::*;

    pub trait Kita<'d> {
        type Wrapped;
    }
    impl<'d, R: Dispatch<Group = GroupA> + 'd> Kita<'d> for Option<R>
    where
        Self: Dispatch<Group = Option<GroupA>>,
    {
        type Wrapped = Self;
    }
    impl<'d, R: Dispatch<Group = GroupB> + 'd> Kita<'d> for Option<R>
    where
        Self: Dispatch<Group = Option<GroupA>>,
    {
        type Wrapped = Option<ManuallyDrop<R>>;
    }
    impl<'d, R: Kita<'d>> Kita<'d> for Option<R>
    where
        Self: Dispatch<Group = Option<GroupB>>,
    {
        type Wrapped = Option<R::Wrapped>;
    }
}

but the name of the attribute cannot be disjoint_impls because it would conflict with existing name