Add a string concatenation operator

[fur] / examples / 22_close_arguments.fur

def make_incrementer(increment_amount) do
  def result(i) do
    increment_amount + i
  end

  result
end

increment_by_one = make_incrementer(1)
increment_by_two = make_incrementer(2)

print(increment_by_one(41), '\n')
print(increment_by_two(40), '\n')