input: Write a Python function to find the number of negative elements in an array.
  assert neg_count([-1,-2,3,-4,-5]) == 4
  assert neg_count([1,2,3]) == 0
  assert neg_count([1,2,-3,-10,20]) == 2
output: def neg_count(arr):
      return sum(1 for x in arr if x < 0)