Feat: implement FixedLengthSum function for sha3

[liyue201]

Description

implement FixedLengthSum function for sha3

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this been tested?

• TestSHA3FixedLengthSum

How has this been benchmarked?

• Benchmark A, on Macbook pro M1, 32GB RAM
• Benchmark B, on x86 Intel xxx, 16GB RAM

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ivokub]

Thanks for the contribution! I refactored a bit for easier reviewing and understanding. I also added tests for different input lengths. Currently there is an issue for input length 0, but I haven't tested other edge cases (at block length) etc. Would it be possible to have a look at the tests and see if there is obvious fix?

[ivokub]

Thanks for the contribution! I fixed an edge case for length=0 and refactored a bit, but otherwise it was perfect!