Fix clippy & use musl target on Rust compiler for static compilation

[dnaka91]

Just some cleanups here and there, mostly related to clippy:

• Fix all existing clippy lints reports for the project.
• Additionally enable clippy::pedantic and rust_2018_idioms lints and fix any reported warnings.
• Update CI to include an extra step for running clippy.

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[dnaka91]

Seems like the ekidd/rust-musl-builder docker image hasn't been updated in a while, hence doesn't include the latest Rust compiler 🤔.

Is it okay for you if I switch to an alternative?

Also, I see you use --target x86_64-unknown-linux-gnu when running in that container but it's a container for building x86_64-unknown-linux-musl. Not 100% sure I understand the purpose. You want to have a fully statically compiled binary right? Then compiling for the musl target should be fine.

[Schniz]

You want to have a fully statically compiled binary right?

Yup, you are right 😃 I don't remember why I needed to use a container. Maybe it's just something I did the same as I did on the ReasonML version

[dnaka91]

Okay I updated the CI job now to simply use the Rust compiler directly.
Didn't need any Docker images for this and tested that the output artifact works in Linux 🎉

[Schniz]

Amazing! thank you!