Start with fresh values for nested sagas

[madebyherzblut]

At the moment the given value for a nested saga already contains the entire path. This leads to a tight coupling of sagas which is not what we want.

This test currently fails

nested_saga_1 =
  Sagax.new()
  |> Sagax.put("c1", assert_value_effect(%{"b" => nil}, {:ok, "c1"}))
  |> Sagax.put("c2", assert_value_effect(%{"c1" => "c1"}, {:ok, "c2"})) # Here the value should only contains the prev comp

nested_saga_2 =
  Sagax.new()
  |> Sagax.put("d", assert_value_effect(nil, {:ok, "d"})) # Since it is the first comp, the value should be nil

saga =
  Sagax.new()
  |> Sagax.put("a", fn -> Log.add(l, "a", {:ok, "a"}) end)
  |> Sagax.put("b", fn -> Log.add(l, "b", nested_saga_1) end)
  |> Sagax.put("f", nested_saga_2)

assert %Sagax{state: :ok, value: value} = Executer.execute(saga)
assert_log l, ["a", "b"]
assert value == %{"a" => "a", "b" => %{"c1" => "c1", "c2" => "c2"}, "f" => %{"d" => "d"}}