Apply some `clippy::pedantic` suggestions

[acuteenvy]

Mostly readability improvements.

• Inlined variables in format! strings
• map_or_else() instead of map().unwrap_or_else()
• Semicolons after expressions that return ()
• copied() instead of cloned()
• String::new() instead of "".to_string()
• Fixed inneficient_to_string
• Removed _ in patterns of type ()

[sharkdp]

Thank you. Let's merge #2755 (and possibly others) first in order not to create too many conflicts.

[sharkdp]

#2755 has been merged now.

[Enselic]

It might sound silly, but can you split this out into separate PRs with simple-to-review and uncontroversial changes please? Right now the PR is a bit too big for my taste I'm afraid.

[acuteenvy]

Sure, each clippy lint in a separate PR?

[Enselic]

The smaller the better as far as I'm concerned (but not too small of course)

[sharkdp]

Let's close this for now.