core: StructValue for attributes, struct Timespan, ParameterLogger overloads, etc.

• add a StructValue to handle attributes represented by a struct
• add ParameterLogger overloads to print std::optionals, std::lists, std::sets and std::maps (just like std::vectors are printed)
• add a struct Timespan (to nstime.h) modeling a time interval with a start time and an end time
• add a function to check if a watchdog has expired (will be used by an upcoming wifi feature)
• fix an uninitialized variable in FatalImpl::FlushStreams()