dicts.py: Added is_dict()

sql.py: DbConn.DbCursor.execute(): When setting self.query, provide default values for query, params because some debug printing statements use self.query directly instead of _add_cursor_info()

sql.py: insert(): Fixed bug where column remaing needed to be removed and sql_gen.Literal value unwrapped

sql.py: mk_insert_select(): cols: Fixed bug where table name needed to be removed from columns with a table

sql_gen.py: ColDict: Only allow dict inputs to constructor so update() can later be used. Ensure keys_table is a Table object.

sql_gen.py: ColDict: Translate NULL values to the key column's actual default value. This avoids incorrectly comparing against NULL in join/filter conditions when the column's actual default value must be used.

sql.py: DbConn: Added col_default()

sql_gen.py: ColDict: Take a db param for later use in getting default column values

sql_gen.py: ColDict: Take a keys_table param for operations that require the output table. Use this to convert all string keys to sql_gen.Col objects.

sql.py: put(): Use a sql_gen.ColDict for row, just like in put_table(), so that put() will benefit from planned enhancements to sql_gen.ColDict, such as translating NULL to the default value of a NOT NULL column