Reduce gas in log256

[TomAFrench]

This PR replaces a multiplication by 8 inside log256(uint256, Rounding) with shifting the bits by 3. There's other usages of multiplication/division through bitshifts in the same file so I don't think this is an example of being overly "optimizooor".

PR Checklist

The change is covered by the existing tests and this changes doesn't warrant docs.

• Tests
• Documentation
• Changelog entry

[frangio]

Do you know how much gas this saves? Waiting to see what our gas reports say.

Regardless, it looks good to me. I would put this on 4.8 (currently in release candidate, feature frozen). @Amxx would you agree?

[TomAFrench]

A MUL is 5 gas whereas SHL is 3 so we should get a saving of 2 gas. If I run forge snapshot I get exactly that saving in practice as well.

[Amxx]

I'm usually against adding non essential thing in the middle of a rc ... and saving 2 gas really doesn't feel essential.

However, we already have a rc update planned, so if you feel like taking this opportunity and adding it, I wouldn't oppose it.

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