Address cargo clippy warnings

[dongsupark]

Run cargo clippy once, and update the code following the suggestions.
Most of them should be straightforwards.

However, a tricky thing is an Error type mismatch like that.

warning: useless use of `format!`
   --> src/bin/download_sysext.rs:440:36
    |
440 |                 None => return Err(format!("Either --input-xml or --payload-url must be given.").into()),
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help:  consider using `.to_string()`: `"Either --input-xml or --payload-url must be given.".to_string()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_format

Changing that to to_string as suggested, the whole main function would fall into a strange state of type conflicts of different error types.
I have seen this issue since days, but so far could not fix that.

[dongsupark]

The error type conflict mentioned here is now resolved in #29.

Will create a separate PR for fixing the other clippy warnings.