Multiple sum tags

This library supports to tag a function with multiple #[sum] attributes, which summarize different impl Trait into different enums.

More syntax of #[sum] and #[variant]

1. #[sum( sum_name => impl Trait )]

2. #[sum( sum_name => impl Trait for Enum )]

3. #[variant( sum_name => variant_name )]

4. #[variant( sum_name => _ )]

The sum_name tells which impl Trait enum the #[sum]/#[variant] belongs to.

#![allow(unused)]
fn main() {
#[sum( ok  => impl Clone )]
#[sum( err => impl Clone )]
fn sum_okeys_and_errors( branch: i32 ) -> Result<impl Clone, impl Clone> {
    match branch % 4 {
        0 => Ok(  #[variant( ok  => _ )] branch ),
        1 => Ok(  #[variant( ok  => _ )] () ),
        2 => Err( #[variant( err => _ )] branch ),
        3 => Err( #[variant( err => _ )] () ),
        _ => unreachable!(),
    }
}
}