Enforce zero-warning policy

[Jacajack]

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Let's clean up all warnings in our code and run cargo clippy. After that we should embrace zero-warning policy on main branch enforced by the CI.

• [x] #290
• [ ] remove all warnings from hdl crate
• [x] remove all warnings from hirn crate

[Jacajack]

@WojciechPtas you can now clean warnings in hdl and add RUSTFLAGS="--deny warnings" env var in the CI