Structural copy/paste

[disconcision]

If code is copied from the editor and pasted in the editor, maybe we should retain the original zipper structure rather than reparsing from text. This would be both faster, and it could retain any metadata not expressed in the text source, like projections.

(May implement this as part of #1218)

Considerations / Challenges:

• Cut/paste could reuse same ids; copy/paste would require replacing them.
• Making sure this doesn't get out of sync with system clipboard