Add code that diffs sub-trees for specified nodes

[sabastiaan]

If we can diff sub-trees for specific nodes than we can verify two properties:

• We can easily implement #220 as each DecoderResult sub-tree should be unique
• We can merge VerifyInstruction nodes which only differ by DecoderResult checks as per #229

More-over, this would also allow for easier debugging and viewing of circuits. Especially if we could color code the dot files based on the diff.