[SHRINKDESC-125] Add exportAsString-esque facility for Node SPI

[rbattenfeld]

Hi Andrew

This feature is requested by Jesper. Can you review it?

I simply added the exportAsString functionality defined for descriptors into the Node class since the existing toString() is not usable IMO.

Thanks
Ralf

[ALRubinger]

As I understand it, this would be a new feature to the SPI only, not to any of the API or *-prototype modules.

Also I think it'd be appropriate to accept some "Formatters" so that the String could have a custom formatting scheme (or one of our own canned ones). We have a very similar example in place in ShrinkWrap Archive.toString(Formatter). So while I completely appreciate this PR, perhaps we could limit the scope and tackle the requirements as above? WDYT?

[rbattenfeld]

I am happy to make inline as it is with the shrinkwrap Archive toString() behaviour. Make sense to me.

[ALRubinger]

Cool. Recommend a new branch based off current upstream/master, then
having another go if you'd like ;)