Use clippy to fix code issues and add it to CI

[Samyak2]

cargo clippy currently emits nearly 50 warnings (and 1 error) on the current code base.

Tasks:

• Run cargo clippy --all-targets --all-features and fix all code improvements suggested by it.
• Add cargo clippy --all-targets --all-features -- -D warnings as a step to CI (after build, test, fmt, etc.) to ensure we have always have a codebase that satisfies clippy. The command was suggested here

[AugustoFKL]

I'm correcting the Clippy issues. Just to let you know, I can't run the code itself and check everything. I'm relying on the tests.

So, if the tests don't cover something, I would appreciate it if you could check it!