[NFC] Fix lints added in new Rust version

[arthurp]

This is a follow-up on #176 which reenables a bunch of lints and fixes code that violate them.

This leaves collapsible_match disabled because it recommends changes that change the behavior of the program (by causing match fall-through that didn't happen before).

Whether or not we want to merge this is up for debate because merging this will cause us to fairly arbitrarily diverge from upstream. Pros: The lints are generally good and will improve the code. Cons: This will create arbitrary conflicts if we try to merge from upstream, however we are not allowed to do blind merges, so that may not matter.

Notes for reviewers:

• The PR is separated into commits, each containing related fixes, so reviewing by commit will mean you are checking related changes together.
• The commits include reformatting the code, so hiding whitespace diffs may helps. (Especially for the collapse ifs commit.)

[arthurp]

@aanyas72 I've added you as a reviewer since you may learn from just reading rust code, and all I need from reviewing this is to check that the change does not affect behavior.