refactor: MerkleRoot#Sum() as MerkleRoot#MustSum()

[bryanchriswhite]

Summary

• Refactors MerkleRoot#Sum() to return an error argument.
• Adds MerkleRoot#MustSum() which behaves as MerkleRoot#Sum() previously did.

Human Summary

AI Summary

reviewpad:summary

Issue

Fixes pokt-network/poktroll#584

We need proper error handling in consumer code where the root has not been checked for validity.

Type of change

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• New feature, functionality or library
• Bug fix
• Code health or cleanup
• Documentation
• Other (specify)

Testing

• Run all unit tests: make test_all
• Run all/relevant benchmarks (if optimising): make benchmark_{all | suite name}

Required Checklist

• I have tested my changes using the available tooling
• I have performed a self-review of my own code
• I have commented my code (godoc format comments see: tip.golang.org/doc/comment)

If Applicable Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have updated any relevant README(s)/documentation and left TODOs throughout the codebase
• Add or update any relevant or supporting mermaid diagrams