Move 'make_row' attribute from Transformer to Cursor

Move 'make_row' attribute from Transformer to Cursor

Having this attribute defined in Transformer protocol will be
problematic when making the RowMaker protocol generic on Row because
we'll then have to also make Transformer generic on Row and
"propagating" the type variable will produce a lot of churn in the code
base.

Also, the variance (covariant) of Row in RowMaker will conflict with that
in Transformer (invariant).

On the other hand, keeping the RowMaker value and its underlying
RowFactory together/attached to the same object (now Cursor) seems
safer for consistency.