Fix clippy::perf and most of clippy::nursery

Part of #7.

This MR fixes any lint in clippy::perf and clippy::nursery (without empty_line_after_doc_comments, option_if_let_else and string_lit_as_bytes):

• empty_line_after_doc_comments because I do not know where the documentation refers.
• option_if_let_else is a big change as it reveals some code smell and should be done last.
• string_lit_as_bytes should be done in a specific MR about write!.