input: Write a function to multiply two numbers using bitwise operations.
  assert multiply(3, 5) == 15
  assert multiply(7, 8) == 56
  assert multiply(0, 10) == 0
  assert multiply(6, 7) == 42
output: def multiply(a, b):
      result = 0
      while b > 0:
          if b & 1:
              result = result + a
          a = a << 1
          b = b >> 1
      return result