Avoid assembly in signature V and S decomposition

[k06a]

Is there any specific reason why assembly was used here?

PR Checklist

• Tests
• Documentation
• Changelog entry

[Amxx]

Not really. It beeing used extensively in the same file might explain how we were drawn to it initially, but it is true we could use native solidity here. I don't expect it to be much more expensive.

[k06a]

I find assembly useful for converting bytes to uint256/bytes32. I see most of the cases in this file are for this purpose.

[frangio]

Thank you!