sql.py: DbConn.DbCursor.execute(): Don't cache ADD COLUMN queries because the rest of the query must be unique in order to use caching

sql.py: add_col(): Don't ignore already-existing columns because sometimes name truncation causes collisions, requiring the caller to version the name. add_index_col(): Version the column name to avoid collisions.

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