input: Write a function to improve the given sum_array function by using Python built-in functions. The function should take in an array of numbers and return the sum of all the numbers.
  assert sum_array([1, 2, 3, 4, 5])==15
  assert sum_array([0, -1, 1])==0
  assert sum_array([])==0
output: def sum_array(arr):
      return sum(arr)