src/lf_apply.rs: clean up usage of masks

[folkertdev]

the commits are self-contained so reviewing per-commit should work well.

[randomPoison]

Ah, it looks like there's been some confusion about tasking! @fbossen is already working on cleaning up lf_apply.rs in #702. Would you be able to clean up one of the other files instead? Any of the "Cleanup c2rust output" tasks that aren't already assigned to someone would be good tasks to start with. Really sorry I didn't communicate that earlier.

[folkertdev]

totally! Depending on what is already done these changes might still be useful, so steal whatever makes sense. (really cleaning up this modules requires more structural changes, so if @fbossen started there there might be little overlap)

[fbossen]

@folkertdev Unfortunately, looks like there isn't anything left to steal. I pushed my current changes to #728. Let me know if I missed something.

[folkertdev]

nothing significant no