Support limited `vec_math()` for clock types

```r
abs(clock::duration_days(-1))
Error: `vec_math.clock_duration()` not supported.
```

^ but calendar types and time points shouldn't support this

``` r
is.finite(clock::year_month_day(2019))
#> Error: `vec_math.clock_year_month_day()` not supported.
```
