tuple support

[braingram]

Should a tuple-like immuntable sequence be added to the ASDF standard?

See:

• ASDF doesn't unpack dict tuple values as tuples asdf#1588
• add note about implicit tuple to list conversion on write/read asdf#1589

[braingram]

To provide a few points for discussion.

What is commonly done for other YAML serializers

If we see ASDF as fancy yaml, it appears that the default behavior for pyyaml and ruyaml is to convert tuples to lists (using their 'SafeDumper's).

Are there other common python yaml libraries?

FWIW: json also converts tuples to lists

How might the introduction of mutable vs immutable types effect other languages

Is there any concept of mutability in YAML?

We are aware of IDL ASDF support. Does IDL have an immutable sequence type (perhaps STRUCTURE?)?