Fix most of clippy::style

Part of #7.

This MR has fixed many of the clippy::style lints, in alphabetical order.

Since the beginning of this refactoring, Rust warnings about dead code have been elided, only focusing on syntactic changes. However, some clippy lints require manual and human checking for functions that are, at the end, unused.

Therefore, this MR has temporary stopped fixing lints from clippy::style and fixed rust lints (except deprecated: will be fixed in another MR).

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The most important commit are 9dd74a86 and 730b2ec9.

Feel free to leave a comment on any function or structure field that you would like to keep. However, since you're modifying traits in a different branch, do not flag the suppressed functions you'll be using: We'll reintroduce them properly later.

Also, let me know if the functions from the dump_borrowck_info.rs file are used for debugging purposes: It might be interesting to move these functions into a specific development mode.