Suggestion: Single `if let` instead of nested `if let { if let`

[pickfire]

Use a single if let instead of a nested one whenever possible:

let value = Some(Err("Some error"));
if let Some(Err(ref error)) = value { // TODO
    return Err(format!("Error parsing 'value': {}", error));
}

Instead of:

let value = Some(Err("Some error"));
if let Some(ref result) = value {
    if let Err(ref error) = *result {
        return Err(format!("Error parsing 'value': {}", error));
    }
}

[oli-obk]

Note that this doesn't work out nicely if fields of structs are accessed, so these should probably be skipped