Fehler integration

[withoutboats]

Takes advantage of fehler's propane integration to make propane
generators Ok-wrapping.

Syntax:

use std::future::Future;
use std::io;

#[propane::generator(throws io::Error)]
async fn foo<T: Future<Output = io::Result<i32>>>(futures: Vec<T>) -> i32 {
    let mut n = 0;
    for x in futures {
       yield x.await?;

       yield n;
       n += 1;
    }
}