sql.py: cast(): Use FOR loop with EXCEPTION block instead of CROSS JOIN with LEFT JOIN to insert entries into the errors table, to get the same optimization benefits this change provides in other filter-out contexts, and to improve clarity

sql_gen.py: NamedValues: Support None cols param for no named columns

sql.py: ensure_not_null(): Made the use of index columns configurable based on the # of rows in the table, because for small datasources, they seem to add 6-25% to the total import time

xml_func.py: _noCV: Fixed bug where assumed items was an iterator when it's now a list

sql.py: add_index_col(), cast_temp_col(): Cache add_col() by providing new comment param to distinguish new columns of the same (colliding) suffixed name but from different source columns

sql.py: add_index_col(), cast_temp_col(): Cache the update that fills in the new column, since it's idempotent

sql.py: update(): Pass cacheable to run_query()

sql.py: add_col(): Added comment param which can be used to distinguish columns of the same name from each other when they contain different data, to allow the ADD COLUMN query (and related queries, such as adding indexes) to be cached

sql_gen.py: Added esc_comment()

sql.py: DbConn.DbCursor.execute(): Allow ADD COLUMN to be cached if it has a distinguishing comment, because then the rest of query will be unique in the face of name collisions