sql.py: Added clean_name()

sql.py: put_table(): Join together input tables into new table for speed and so don't modify input if values edited

sql.py: mk_flatten_mapping(): Take as_items param to return a list of dict items instead of a dict. Sort preserve cols before other cols. flatten(): Turn on as_items so that cols list is sorted in input order, with preserve cols first. This ensures that if a pkey is provided in preserve, it will be the first col in the generated table.

sql.py: mk_flatten_mapping(), flatten(): Take list of cols to select instead of using all cols in all tables to join

sql.py: mk_flatten_mapping(), flatten(): Renamed flat_table param to into to be consistent with run_query_into() and put it first because it is the output param

sql.py: Added flatten()

sql.py: mk_flatten_mapping(): preserve Col objects will have tables changed to flat_table to work with flattened table

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