Clippy warnings (Fix them all and add a CI job for it)

[ogoffart]

The code is full of clippy warning. It would be nice to clean up a bit. Especially the part in generated code, because our user shouldn't get clippy warnings in their code because they are using Slint.

Ideally it should be tested in CI than there is no warnings in the generated code (by, eg, running clippy on the rust test)

Note: some common sense should be applied when fixing some warnings. Not all suggestions from clippy makes sense, and sometime it's ok to just ignore them. Performance suggestions almost always make sense, or the ones about using function that we maybe did not think of while writing the code. But style ones are sometimes a bit dubious.