A method for iterating over causes

[epage]

Currently there is cause and root_cause.

It'd be nice to have something like iter_causes to help in cases like this

println!("Error: {}", err);
let mut cause = err.cause();
while let Some(c) = cause {
    println!("Caused by: {}", c);
    cause = c.cause();
}

to

println!("Error: {}", err);
for cause in err.iter_causes() {
    println!("Caused by: {}", cause);
}

[withoutboats]

I'm open to it, but I think not in 0.1. For now the while let loop is not so bad.

[epage]

I'm fine with it being a lower priority

[withoutboats]

#54 resolved this issue :)