We current access the internals of the bson_iter_t structure (see CDRIVER-2467 for details). Once the C driver provides a mechanism to address this, use it.]

Let's add a libbson function bson_iter_keylen which returns the length of the key. This is already known, it's something like "key - d1". The C++ iterator can store this length before destroying the bson_iter_t, then pass it back into a new libbson function like bson_iter_init_with_data. That will end up doing only one key-length calculation per iterator increment, whereas right now the C++ Driver does two.