Extend Tuple interface with ordered field access

Let's extend Tuple interface by adding methods for indexed column access (like JDBC resultset has).

It may need to expose more information about Tuple structure, such as

• column name -> column index
• all columns in the tuple (name + type (ColumnType))
• length()
• value(int index)
• Iterable implementation

This may be useful for SQL\JDBC drivers and bulk operation where Tuples can have the same structure and column order.