We need to sample n random ones from AllStructs. We can sample that
without keeping first whole AllStructs result in memory.

Starting from maxdepth=4 and nkeys=10 there are too many entries in that
whole list. For example the following input was causing treegen
allstructs to be OOM killed on my machine:

    4 2 20/8808469856591574482 0:f,1:d,2:g 0:d,1:e,2:a,3:h,4:d,5:h,6:d,7:c

Now it works ok without eating memory.