Version graph fixes

[Sushisource]

What changed?

• Don't allow removing nodes. It opens up too many confusing questions about what to do with open workflows, and the effect of removing a node can be accomplished by just adding a new version.
• Fix some straight up bugs

Why? Bugs bad, obviousness good.

How did you test it? Many new tests, improved fuzzer

Potential risks nope

Is hotfix candidate? no