input: Write a python function to implement an ordered dictionary with methods to set and get items, and to get its length.
  assert len(OrderedDict()) == 0
  od = OrderedDict()
  od['a'] = 1
  assert od['a'] == 1
  assert len(od) == 1
  od['b'] = 2
  assert od['b'] == 2
  assert len(od) == 2
  od['a'] = 3
  assert od['a'] == 3
  assert len(od) == 2
output: def OrderedDict():
      class OrderedDict:
          def __init__(self):
              self._keys = []
              self._values = {}
  
          def __setitem__(self, key, value):
              if key not in self._keys:
                  self._keys.append(key)
              self._values[key] = value
  
          def __getitem__(self, key):
              return self._values[key]
  
          def __len__(self):
              return len(self._keys)
  
      return OrderedDict()