Added structs

[fur] / examples / 17_short_circuiting.fur

def return_zero() do
  print('This should not print\n')
  2
end

def return_one() do
  print('This should only print once.\n')
  1
end

def return_true() do
  print('This should print exactly twice.\n')
  true
end

def return_false() do
  print('This should print exactly twice.\n')
  false
end

0 < return_one() < 2
0 < 0 < return_zero()
return_true() or return_true()
return_false() and return_false()