Fix several `clippy::pedantic` lints

[adriandelgado]

I ran cargo clippy --fix -- -W clippy::pedantic and manually inspected the changes. These are the fixes. There are a lot of fixes left, but this is the first batch. Also, while reading the code I found some bug fixes for future PR. (and performance improvements too). Sometimes, each programmer has their own code style so feel free to revert any changes this PR made.

This is a summary of the changes:

• Use backticks around rust_xlsxwriter and other crate elements
• Use underscores as separators in numbers and hex colors
• Avoid star imports
• Remove redundant () after names of functions and methods
• Change "".to_string() and String::from("") to String::new()
• Use From::from instead of as conversions whenever possible
• Add #[must_use] to relevant functions
• Change Iterator::cloned to Iterator::copied whenever possible
• Use sort_unstable instead of sort when working with primitives
• Change for _ in _.iter() to for _ in &_ (same with iter_mut)

[adriandelgado]

As said in the related issue feel free to ask to revert any of the changes suggested. And this was just the first step to get familiarized with the code, I plan to submit less trivial PRs in the near future.

All of the changes proposed here are reasonable (in my opinion), I can give a justification for any of them. I can also split this PR into several parts if you want to.

[adriandelgado]

My bad, I didn't re run the tests after rebasing...

[adriandelgado]

I added the missing &mut and removed some backticks. It now passes cargo doc and cargo test.

[adriandelgado]

I ran cargo clippy --all-targets in addition to cargo doc and cargo test. It will pass everything now. I think.

It seems like some of the iterator fixes made clippy find other warnings.