[Solution] Vector<T>::contains() not implemented

Fortunately this is a templatized function and isn't needed by the compiler until it's needed by the user.

I've implemented it in vector.h as follows:

template<class T> inline bool Vector<T>::contains(const T& t) const
{
  return find_next( t ) != -1;
}

I'll try to get this into the git dev baseline as soon as I can.