sql.py: mk_flatten_mapping(): Added preserve param for list of columns not to rename

sql.py: esc_name_by_module(): Support module value None, and use default module psycopg2 for it

sql.py: put_table(): Renamed *pkeys_ref to *pkeys to reflect that they are now objects rather than an array-based references

sql.py: run_query_into(): Renamed into_ref param to into to reflect that it's now an object rather than an array-based reference

sql.py: run_query_into(): Made into_ref a sql_gen.Table instead of an array containing a table name to improve flexibility and clarity

dicts.py: Added join()

sql.py: Added mk_flatten_mapping()

sql.py: put_table(): Renamed the copy of in_tables that gets modified to in_tables_, so that the original list can eventually be reused in joining together the input tables into a temp table

sql.py: run_query(): FunctionValueException: Also match "date/time field value out of range" errors

sql.py: put_table(): conds: Use a set instead of a list for faster checking of the "cond not in conds" assertion