introduced an internal phongo_bwc_extract_id method based on php_phongo_bulkwrite_extract_id. This can be refactored to use a common utility function.

Additionally, the original approach in php_phongo_bulkwrite_extract_id that converts the entire document can be rewritten to iterate BSON and use phongo_bson_value_to_zval.

Targeting this change for 2.0 would mean that we need not prefer phongo_bson_value_to_zval_legacy for the original php_phongo_bulkwrite_extract_id code path. It would be acceptable to return document values for _id as a Document instance instead of stdClass (admittedly an edge case).