maths/number_of_digits.py: Streamline benchmarks (#7913)

* maths/number_of_digits.py: Streamline benchmarks

```
num_digits(262144): 6 -- 0.2226011250168085 seconds
num_digits_fast(262144): 6 -- 0.13145116699161008 seconds
num_digits_faster(262144): 6 -- 0.09273383300751448 seconds

num_digits(1125899906842624): 16 -- 0.6056742920191027 seconds
num_digits_fast(1125899906842624): 16 -- 0.15698366600554436 seconds
num_digits_faster(1125899906842624): 16 -- 0.1027024170034565 seconds

num_digits(1267650600228229401496703205376): 31 -- 1.1957934170495719 seconds
num_digits_fast(1267650600228229401496703205376): 31 -- 0.15552304196171463 seconds
num_digits_faster(1267650600228229401496703205376): 31 -- 0.13062308297958225 seconds
```

* updating DIRECTORY.md

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* Update number_of_digits.py

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