sql.py: add_index(): Version index names to avoid collisions

sql.py: DbConn.DbCursor.execute(): Always cache structural changes, not just if they throw an exception

sql.py: add_index(): Fixed bug where expr could not be deep-copied until ensure_not_null() had been run on it, because ensure_not_null() modifies the index_cols of the table and this needs to be modified on the original table

sql.py: add_index(): Fixed bug where expr could not be deep-copied until ensure_not_null() had been run on it, because ensure_not_null() modifies the index_cols of the table and this needs to be modified on the original table

sql_gen.py: EnsureNotNull: Added to_str() which uses the column's index col if available. This works around a PostgreSQL bug where indexes on expressions are sometimes not used, but indexes on plain columns are used.

sql_gen.py: Store index cols in the table instead of in the column, because columns are often recreated from the table and name but tables are generally not copied

sql_gen.py: Added is_indexed_col()

sql.py: add_index(): Use new sql.ensure_not_null(), which creates a separate column to store the index expr where possible

sql.py: Added add_not_null(), add_index_col(), ensure_not_null()

sql_gen.py: Added is_temp_col()