optimise speed

This is taking way too long for my liking.

```r
 design("Photosynthesis") |> 
  set_units(day = 1:5,
            row = 1:4,
            col = 1:24,
            well = crossed_by(row, col),
            run = 1:33,
            temp = c(26, 36),
            sample = crossed_by(day, well, run, temp)) |> 
  serve_table()
```