Move the row maker as a Transformer attribute

Move the row maker as a Transformer attribute

Instead of carrying the row maker (_make_row attribute) on the cursor
and possibly calling it to transform each row in cursor methods, we
define a 'make_row' attribute on Transformer that is possibly used in
load_row() and load_rows().

In the Python implementation of Transformer.load_rows(), we use tuple as
as make_row() when the attribute is unset.

In the Cython implementation, we make 'make_row' a plain property with a
'_make_row' attribute under the hood. We finally transform individual or
list of records using self.make_row().