Fallback as impl Trait

Sometimes the downstream users do not bother "poluting" signatures of functions which call upstream #[cex] APIs returning checked exceptions. If all variants of the error Enum!() have implemented some famous trait, e.g. std::error::Error, the downstream users get a chance to simply write -> Result<_, impl std::error::Error> in their function signatures.

Example

#![allow(unused)]
fn main() {
use std::error::Error;
use enumx::export::*;
use enumx::predefined::*;
use cex::*;

impl Error for A { /* omitted */ }
impl Error for B { /* omitted */ }
impl Error for C { /* omitted */ }

#[cex] pub fn some_cex_function() -> Result!( () throws A, B, C );

fn downstream() -> Result<(), impl Error> {
    Ok( some_cex_function()? )
}
}