sql_gen.py: with_default_table(): Added overwrite param to overwrite the table (if it isn't a NamedCol)

sql_gen.py: Join.to_str(): join(): Get just the table name of left_table and right_table using `.to_Table()`. Moved order switching of tables inside join() because the order reversing only applies to an individual condition.

sql_gen.py: Renamed set_default_table() to with_default_table() and copy col before modifying it so don't modify input

sql_gen.py: Added set_default_table(). as_ValueCond(): Use set_default_table() instead of as_Col() so that any name-only column also gets its table set. Join.to_str(): Parse left side using set_default_table() instead of as_Col() so that any name-only column also gets its table set.

sql_gen.py: Join: mapping param defaults to {} for e.g. CROSS JOINs. to_str(): Omit join_cond if mapping is empty, rather than if join is a specific type.

sql_gen.py: NamedValues: Change cols to Col objects with the table set to `name`

sql_gen.py: Added set_cols_table()

sql.py: mk_insert_select(): returning: Use sql_gen.to_name_only_col()

sql_gen.py: NamedTable: cols: Use sql_gen.Col objects or name strings instead of pre-rendered SQL code

sql_gen.py: NamedTable: Wrap nested code in Expr if needed