fr::HttpSocket has been removed.
fr::Sendable has been added. Classes can inherit this if they want to be sendable through sockets. Both fr::Http (which is inherited by fr::HttpRequest and fr::HttpResponse), and fr::Packet inherit this, to allow them to be sent through fr::Socket::send()/fr::Socket::receive().
Fixed broken fr::SocketSelector and fr::SocketReactor, as they no longer accepted fr::Listener's due to fr::Listener no longer inheriting fr::Socket.
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