On Windows, unlike on Mac or Linux, boost::ptr_map<> started insisting on this
concept of clonability. In other words, it wants to own a unique instance of
the pointee; if you copy a value_type -- even to dereference an iterator! --
it wants to construct a whole new instance of the mapped_type. That's nuts. A
std::map<..., boost::shared_ptr<>> has the property I want (the mapped_type
goes away when the entry is erased), plus it's willing to pass around the
shared_ptr to the same instance of the mapped_type. This change also permits
simplifying a couple awkward kludges I'd already had to make to accommodate
ptr_map's idiosyncracies.