Explore clippy rules and apply quality of life recommendations

[ebuchman]

This is an issue to track the work started in #122 cc @xla

[liamsi]

It seems like in the change from circleci to github actions clippy is called in a less strict way than perviously. If I run it locally via:
cargo clippy --all --all-targets -- -Dwarnings -Drust-2018-idioms it often shows some errs that do not show up in CI. Would be good to start with this simple online change.

[thanethomson]

Is the goal here to effectively apply as many of the clippy::pedantic rules as possible?

And what's the "definition of done" for this issue? It seems a little vague.

[greg-szabo]

Definition of done, based on the issue: turn all clippy warnings into errors.

I understand that there's an underlying bigger work that happened here previously (to apply as many clippy rules as we can) but I don't see this work progressing forward right now and half-defined issues won't help it either.

I propose we close this issue when we close #406 and if/when we feel the need to improve our clippy settings again, open an issue with well-defined targets.