JSON Type named `tup` elements

[streamich]

• Consider adding ability to name tup elements.
• Alternatively, introduce new vec type, which is similar to tup, but supports named elements, and possibly optional trailing elements.
• Alternative, reuse the existing obj type. As the fields are already sorted, add ability to serialize obj as a tumple.
  • Consider adding format field to obj type. Say, format: "tup".