Record aggregate state while processing Multi (on top of our fork)

[joaobalsini]

Before this PR, a user could only compare aggregate state before the multi start with the aggregate state on the current step.

This PR allows a user to record aggregate state under a name for each of the multi steps (after the step has been processed in the aggregate) and later compare it with the aggregate state before the multi started or after any of the previous steps.

This helps on some complex cases multis, in which we need to know what changes were applied on previous steps of the multi.

This code change doesn't deprecate current functionality.

Example

      aggregate
      |> Multi.new()
      |> Multi.execute(:interesting_event, fn aggregate ->
        %Event{data: 1}
      end)
      |> Multi.execute(fn aggregate, %{interesting_event: _aggregate_after_interesting_event} ->
        %Event{data: 2}
      end)