Fix three issues related to deepcopying elements

[tbennun]

This PR fixes #1439 and #1443 by adapting fields and the deepcopy operation for states:

1. Skips derived field parent being set if a state is deepcopied on its own
2. Does not add a new field to AST nodes during preprocessing. That parent-pointing field outlives preprocessing and ends up copying the entire original AST for short codeblocks.
3. Does not add a new field to states during state propagation.

[phschaad]

LGTM, thanks