Retrieving with optional of reference wrapper

[ftschirpke]

This PR merges the feature branch that was intended for the implementation of functions which would provide tools to deal with the different types of effects providers.
While implementing this I starting changing a lot of the content libraries' APIs.
Previously, one would first ask the library if something exists, and then get it, because get() would throw if the object wouldn't exist.

if (library.contains(item_name)) {
    const auto& item = library.get(item_name);
}

With the new approach, using std::optional in combination with std::reference_wrapper, only one hash has to be evaluated and we still have the security of references.

auto item_optional = library.get(item_name);
if (item_optional.has_value()) {
    const auto& item = item_optional.value();
}

This approach still has few problems, but I am going to merge it for now and open an issue about addressing these issues.