Fix all clippy warnings and add clippy to the CI

[koute]

Clippy doesn't like the theoretically redundant closure calls that I've added in my last PR, and since clippy doesn't run on the CI here I didn't notice.

While I was at it I also fixed all other clippy warnings, plus one error as it was complaining about an unsafe that I've now removed. I've also added clippy to the CI (hopefully it works).

Fixes #454

Fixes #456

[ggwpez]

Maybe we should add a Clippy config file?
Could be the one from Substrate in .cargo/config.toml.

[ggwpez]

Looks good, just some Qs.

[koute]

Maybe we should add a Clippy config file? Could be the one from Substrate in .cargo/config.toml.

Hmmm.... I think we should be fine with the default one here, as this is orders of magnitude smaller so we don't need to disable a bunch of lints like in Substrate.

[ggwpez]

Hmmm.... I think we should be fine with the default one here, as this is orders of magnitude smaller so we don't need to disable a bunch of lints like in Substrate.

Its not just disabling, it is also denying some. But only correctness and complexity as it seems.
Up to you 🤷 Just denying those is probably enough.

[koute]

Added a fix + test for #456 and also fixed clippy lints in the derive crate (this was as far as I can see necessary to test the fix for #456). Should be good to go once the CI passes.