[proofs] Adding post-visit processing to proof node updater (#8431)

[proofs] Adding post-visit processing to proof node updater (#8431)

Previously in the proof node updater only structural reasoning was done at post-visit time (merging subproofs, checking free assumptions). This commit adds a virtual method to allow updating the proof node at post-visit time.

In particular this commit is important to functionality in the alethe post processing that is deactivated in master.

CC @Lachnitt