`std::iterator` is deprecated

[adamnovak]

In C++17, inheriting from std::iterator is deprecated, on the theory that code is actually better if it just explicitly defines all the typedefs that std::iterator would get you by inheritance.

But it is still used e.g. here: https://github.com/rizkg/BBHash/blob/1803c2325afab8ad045c94ef7872a319bc44a5e5/BooPHF.h#L45

This leads to compiler warnings when compiling under C++17 or later.