Fix hash<IPv4/6Address> build issues in VC

* fixed compile failure due to macro

* add functional to cxxstd.h to try to fix mac

* clang bug identified, moving functional include to later

* last step, move hash def to header

* avoid allocation on hash

* set ipv6 back to string hash

src/ipv6_address.cpp

@@ -152,13 +152,3 @@ IPv6Address operator&(const IPv6Address& lhs, const IPv6Address& rhs) {
 }
 
 } // Tins
-
-#if TINS_IS_CXX11
-namespace std {
-
-size_t hash<Tins::IPv6Address>::operator()(const Tins::IPv6Address& addr) const {
-    return std::hash<string>()(addr.to_string());
-}
-
-} // std
-#endif // TINS_IS_CXX11