Issues with `%/%` and `%%` and division by `0`

```r
duration_years(5) %/% 0
duration_years(5) %/% duration_years(0)

duration_years(5) %% 0
duration_years(5) %% duration_years(0)
```

These are all undefined C++ behavior and crash the session, but we should return missing values

In particular we should be consistent with `2L %/% 0L = NA` rather than `2 %/% 0 = Inf` since infinite durations are not supported