Well, even powers are completely unintuitive with your suggestion. What distribution would you expect when using `pow($x, 4);`? Are the low, the medium or the high numbers favored? Think a moment about it.

I would say: The high ones, but after thinking about it one will see that in fact the lower numbers are favored!

Therefore I will not change the actual algorithm, unless the distribution stays (roughly) the same. And the numbers show that depending on the chosen function the biased generator is between 25% and 16% of the speed of `numberBetween`, there are worse generators (e.g. `numerify`, which generates each digits independently).

If I could convince you I will happily fix the test failures and rename the function and whatnot, but if you still think that the algorithm is too slow then you may close the pull request.