Moving all forward declarations into a common header file, rosa/core/forward_declarations.h; moving the definition of message_t into forward_declarations.h; defining a type alias id_t in forward_declarations.h and use it as the type of Unit identifiers; refactoring the static member function createUnit of System so that the actual System subclass calling the function can be used as the actual type instead of System to instantiate a Unit (subclass) with.