Deprecate use of boost::shared_ptr in favour of std::shared_ptr

Assuming `boost::shared_ptr` and `std::shared_ptr` are distinct types, let Boost.Python provide builtin support for `std::shared_ptr` whenever the compiler supports it, and optionally support for `boost::shared_ptr` (for backward compatibility). The latter may be removed later, to reduce Boost.Python's dependency on Boost.