Enable logging of inner heuristics

[Saethox]

From my understanding, we currently run into problems if we want to log e.g., a nested local search. A nested heuristic requires a new Scope, because it may use the same custom State as the high-level heuristic, for example Iteration. But the current implementation of State::with_substate discards the substate after executing the given function, including the Log. Correct me if I'm wrong, @luleyleo.

A way to aggregate nested Log's would therefore be appropriate.