input: Write a function that improves the given function by using Python built-in functions. The original function calculates the sum of all numbers in an array by iterating through the array and adding each number to a total. The improved function should achieve the same result using a built-in function.
  assert sum_array([1, 2, 3, 4])==10
  assert sum_array([5, 10, 15, 20])==50
  assert sum_array([-1, -2, -3, -4])==-10
  assert sum_array([0, 0, 0, 0])==0
  assert sum_array([])==0
output: def sum_array(arr):
      return sum(arr)