`arithmetic_side_effects` should consider the conversion of primitives

The right-hand-side comes from an `u8` that will never overflow when adding `86400`.

```rust
use core::time::Duration;

#[deny(clippy::arithmetic_side_effects)]
pub fn from_days(days: u8) -> Duration {
    Duration::from_secs(86400 * u64::from(days))
}
```