Decode a BSON string to document_class.

def _bson_to_dict(data, opts):
    """Decode a BSON string to document_class."""
    try:
        obj_size = _UNPACK_INT(data[:4])[0]
    except struct.error as exc:
        raise InvalidBSON(str(exc))
    if obj_size != len(data):
        raise InvalidBSON("invalid object size")
    if data[obj_size - 1:obj_size] != b"\x00":
        raise InvalidBSON("bad eoo")
    try:
        return _elements_to_dict(data, 4, obj_size - 1, opts)
    except InvalidBSON:
        raise
    except Exception:
        # Change exception type to InvalidBSON but preserve traceback.
        _, exc_value, exc_tb = sys.exc_info()
        reraise(InvalidBSON, exc_value, exc_tb)