Boost only implements it iself starting from version 1.74, but a specialization
of std::hash<> can be added trivially to allow the use of
std::unordered_set<boost::intrusive_ptr<T>> and
std::unordered_map<boost::intrusive_ptr<K>, V>.
Being unable to use such types already came up a few types in the past, often
resulting in the use of raw pointer instead which always involves an additional
"is this safe?"/"could the object go out of scope?" discussion. This commit
simply solves this for the future by simply allowing the use of intrusive_ptr
in unordered containers.
