input: Write a function to search for a target value in an array and return its index or -1 if not found.
  assert search([1, 5, 10, 15, 20], 15) == 3
  assert search([1, 5, 10, 15, 20], 10) == 2
  assert search([1, 5, 10, 15, 20], 25) == -1
output: def search(arr, target):
      for i, value in enumerate(arr):
          if value == target:
              return i
      return -1