Speed up `assign_trts()`

[emitanaka]

The design below takes about 13 minutes to run:

design("Mario's plant pathologist problem") %>% 
  set_units(rep = 3,
            col = 12, 
            row = 12, 
            # so 3 x 12 x 12 pots in total?
            pot = crossed_by(rep, col, row),
            # 4 varieties fit per pot so 4 units per pot?
            unit = nested_in(pot, 4)) %>% 
  set_trts(barley = 576) %>% 
  allot_table(barley ~ unit)

[emitanaka]

Note: this took 1 hour for the specs shown below so it's unacceptably long for those with older computers.