This changes WriteResult to encapsulate a bson_t, which is populated by mongoc_bulk_operation_execute(), instead of the private mongoc_write_result_t struct. This entailed significant changes to the WriteResult debug handler, so new tests have been added for it.
With phongo_execute_write() modified to populate a bson_t reply, we also changed error handling to pull a BulkWriteException message from bson_error_t (PHPC-626). That change required several error tests to be modified for the new message format.
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