Method on_document_begin

bench/bench.cpp:505–505

            constexpr static std::size_t max_string_size = std::size_t(-1);

            bool on_document_begin(system::error_code&) { return true; }
            bool on_document_end(system::error_code&) { return true; }
            bool on_object_begin(system::error_code&) { return true; }
            bool on_object_end(std::size_t, system::error_code&) { return true; }