ex 3.80 - impl
ex 3.80 - try to draw the function.
ex 3.80 - done
normal-order evaluation
3.5.5 - rand num stream impl
new lib function: split = take + drop
3.5.5 - done
ex 3.81 - impl
ex 3.81 - done
ex 3.82 - done
ex 3.82 - use random stream

I think this should finally be the right approach.
The previous one uses `random-in-range` which has
side effects and does not respect the idea of seeing the random
numbers as a stream of numbers.
extra task - bank account stream representation