Semi-random selection of clippy topics that were
uncontroversial before:-).
Leaves about 84 deduplicated issues in the code base
according to clippy from nightly.
Add some code to do platform-independent path processing.
This is necessary aas WASM does e.g. not have any absolute paths and
such and the compiler tended to produce wrong results in that case.
Side-effect: We no longer need to depend on `dunce`
