Ptr improvements

This gets rid of the boost::any since all that's needed is a simple pointer to any LiveCellRef<> type. That and the string also incurred heap allocations every time a Ptr was copied or constructed with a LiveCellRef. They've been cleared up so creating/copying Ptr objects themselves should be much more efficient.

I've been considering storing a Class* in the LiveCellRefBase also, so it only needs to be looked up once per LiveCellRef creation instead of every time Class::get(ptr) is called.