Add clippy suggestions

[kevaundray]

Made a few clippy fixes in this PR. I did not do all of the clippy fixes as it may be that only certain things should be changed and or clippy fixes are not deemed necessary.

This PR is more of a temperature check :)

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[jameysharp]

In general we don't use Clippy because there's a general consensus that it's too opinionated. That said, I like most of these changes!

There are a few that I would like for you to remove from this PR, and ideally arrange that Clippy will ignore them in the future. (I assume you can add something to the top-level clippy.toml but I haven't looked into Clippy's configuration.)

With these changes, I'd be happy to merge this. Thanks!

[kevaundray]

Thanks for the review! It seems that we can't use Clippy.toml to enable lints globally.

There was a workaround done by Embark, which is to put it in the .config/Cargo.toml file -- I can open another issue for this, as it would require removing the lints from each lib.rs file and putting them in .config/Cargo.toml

[jameysharp]

Yeah, let's not bother with trying to configure Clippy then. We really don't want to spend much time on Clippy lints.

That said, I'll go ahead and take these changes. I do think they are improvements.

I'm sure there are more improvements we could find by digging through Clippy's suggestions, but that isn't something we want to spend review time on.

We have a bunch of open issues that we'd love your help with. Take a look at cranelift:E-easy Issues suitable for newcomers to investigate, including Rust newcomers! and see if any of those sounds interesting to you!