We should get rid of the logarithm taking a base. Just have 3 functions. log (implicitly base e), log10, and log2. That's all the bid128 library gives us anyway. and these are names which match std:: terminology.
No need to get clever about it. Any other bases (very uncommon) would be accomplished by dividing logs. That's exactly what it does internally anyway. Why does naturalLogarithm take signalingFlags but the wrapper logarithm doesn't? That's also inconsistent.

This is like SERVER-91935, and addresses the same API clarity problem but with a different solution.