Fix most of clippy::complexity and clippy::correctness

Part of #7.

This MR fixes any lint in clippy::complexity (without clippy::useless_format) and clippy::unused_io_amount (without clippy::correctness):

• clippy::useless_format has revealed a discussion about the error type (currently String and should be an enumeration).
• clippy::unused_io_amount is a big change and will be done in a separate MR.

Also, one function got removed since unused (see 6f684890)