Node identity

[xatapult]

This comes from discussions in https://github.com/xproc/3.0-steps/issues/395 and https://github.com/xproc/3.0-steps/pull/402. The issue is however, I think, more general.

I think we should put some discussion effort in trying to define "node-identity" in XProc. For instance:

• Is there node-identity between a node passed through an option and one in an input document if both come from the same source document?
• is there node-identity if the same document arrives on multiple ports of a step?
• What if we put a node in a variable? Does that break node-identity? What if this variable is passed into an option?

I suppose other X specs already say something about it but I haven't investigated yet.

[xatapult]

Issue is here so we won't forget to discuss this if/when there will be a Vnext.