perf: load results by row rather than by column

perf: load results by row rather than by column

In the past we were converting results to Python tuples proceeding
column by column. The rationale was that certain overhead such as
selecting the loader was to be paid only once per column, not once per
datum.

However this analysis was dismantled in #1163, see especially the comment at
https://github.com/psycopg/psycopg/pull/1163#issuecomment-3288921422
for some benchmark comparing various conversion strategies. In the end,
the simple row-by-row loading in a single function ends up being more
performing. Performance now surpasses the one of psycopg2.

See also #1155 for an initial analysis of performance regression.

A big thank you to Jörg Breitbart (@jerch) for this improvement!