std::reference_wrapper for ModManager

[wvpm]

T* is used throughout our codebase to store pointers to objects whenever T& isn't viable due to move semantics.
For example vector_ordered_set<Mod const*>. Technically these pointers could be null and to be safe you'd have to check it isn't null before dereferencing.

Often we know the pointer may never be null. std::reference_wrapper<T> is a great way to communicate that. It is implicitly convertible to T&. This allows us to do for (Mod const& mod : loaded_mods) (with memory::vector<std::reference_wrapper<const Mod>> loaded_mods).

This approach does require the type is hashable and equatable. For types derived from HasIndex, the index is used for this. For types that don't derive from HasIndex, but do derive from HasIdentifier, the identifier is used.