Add combine_latest function

ahh... I see what you were doing before.... this is broken now... it was working before.

Here's what I had in mind:

```elixir
Stream.scan({nil,nil}, fn({tag, el}, {a, b}) ->
  case el do
    {1, value} -> {value, b}
    {2, value} -> {a, value}
  end
end
```