Remove misleading bls comments

[jtraglia]

📝 Summary

• Fix typo in contributing guidelines document.
• Remove comments from pk/sig *FromBytes functions.
  • I believe that the SignatureFromBytes comment is misleading.
  • The public key and signature are big-endian, right? (see reference)
• Use easier to read BLST types which are defined at the top of the file.
• For sanity, add a signature verification test, stolen from Teku.

⛱ Motivation and Context

When looking over this file, the little-endian/big-endian difference seemed odd.

📚 References

• https://ethresear.ch/t/bls12-381-keys-and-signatures-sizes/6152/3

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✅ I have run these commands

• make lint
• make test
• go mod tidy

[metachris]

thank you!