Merge node

[Stevengre]

Blocked on: #4617
Blocked on: #4621
Blocked on: #4612

This pull request addresses issue #4425 and refactors #4521 for simplification and ease of review.

• The key implementations are in kcfg.py and semantics.py.
• Unit tests are located in pyk/src/tests/unit/kcfg/test_merge_nodes.py.
• Changes in other Python files are either adaptations for Edge or result from the rebase. I am uncertain how to eliminate these changes caused by rebase.

[Baltoli]

@Stevengre you'll want to use git rebase -i develop to interactively select the commits that contain your changes, and to drop the ones that don't belong to this PR. This is a useful git workflow to know how to do in general - there's a good guide here if you're not familiar.

[Stevengre]

Thank you so much! I'll try it!

[ehildenb]

Subsumed by: #4628