Replace `ceil(log(..., 2))` with `frexp`

Lua 5.1 does not support passing a base; it would use the natural logarithm, thus the values would be slightly off.

Notes:

• Found with https://gist.github.com/appgurueu/21132254211f32fb210c429cf56c4b2b
• Untested
• Alternative would be continuing using math.log but dividing by math.log(2) (basis transformation); frexp is IMO more readable and probably more efficient (though probably negligibly so) - use whichever you consider more readable