adding Ln benchmark
adding Ln benchmark

Adding a Ln benchmark that varies the output precision as well as the scale and
number of digits of the input number. This will guide any performance
improvements. The intention is to add more benchmarks using the same runBenches
helper.

Local results for Ln:
```
Ln/P2/S-100/D2-4       842µs ± 0%
Ln/P2/S-100/D10-4     6.31ms ± 0%
Ln/P2/S-100/D100-4    29.8ms ± 0%
Ln/P2/S-10/D2-4        370µs ± 0%
Ln/P2/S-10/D10-4       580µs ± 0%
Ln/P2/S-10/D100-4     23.7ms ± 0%
Ln/P2/S-2/D2-4         449µs ± 0%
Ln/P2/S-2/D10-4        645µs ± 0%
Ln/P2/S-2/D100-4      25.5ms ± 0%
Ln/P2/S2/D2-4          381µs ± 0%
Ln/P2/S2/D10-4         523µs ± 0%
Ln/P2/S2/D100-4       75.4ms ± 0%
Ln/P2/S10/D2-4         480µs ± 0%
Ln/P2/S10/D10-4        557µs ± 0%
Ln/P2/S10/D100-4      30.5ms ± 0%
Ln/P2/S100/D2-4       1.09ms ± 0%
Ln/P2/S100/D10-4       805µs ± 0%
Ln/P2/S100/D100-4     89.2ms ± 0%
Ln/P10/S-100/D2-4      768µs ± 0%
Ln/P10/S-100/D10-4     866µs ± 0%
Ln/P10/S-100/D100-4   29.3ms ± 0%
Ln/P10/S-10/D2-4       608µs ± 0%
Ln/P10/S-10/D10-4      688µs ± 0%
Ln/P10/S-10/D100-4    67.7ms ± 0%
Ln/P10/S-2/D2-4        670µs ± 0%
Ln/P10/S-2/D10-4       556µs ± 0%
Ln/P10/S-2/D100-4     26.7ms ± 0%
Ln/P10/S2/D2-4         552µs ± 0%
Ln/P10/S2/D10-4        521µs ± 0%
Ln/P10/S2/D100-4      26.6ms ± 0%
Ln/P10/S10/D2-4        558µs ± 0%
Ln/P10/S10/D10-4       748µs ± 0%
Ln/P10/S10/D100-4     50.6ms ± 0%
Ln/P10/S100/D2-4      1.06ms ± 0%
Ln/P10/S100/D10-4      948µs ± 0%
Ln/P10/S100/D100-4    26.8ms ± 0%
Ln/P100/S-100/D2-4    26.4ms ± 0%
Ln/P100/S-100/D10-4   28.1ms ± 0%
Ln/P100/S-100/D100-4  34.0ms ± 0%
Ln/P100/S-10/D2-4     25.9ms ± 0%
Ln/P100/S-10/D10-4    48.6ms ± 0%
Ln/P100/S-10/D100-4   28.2ms ± 0%
Ln/P100/S-2/D2-4      23.3ms ± 0%
Ln/P100/S-2/D10-4     26.5ms ± 0%
Ln/P100/S-2/D100-4    33.0ms ± 0%
Ln/P100/S2/D2-4       30.0ms ± 0%
Ln/P100/S2/D10-4      26.9ms ± 0%
Ln/P100/S2/D100-4     20.7ms ± 0%
Ln/P100/S10/D2-4      29.8ms ± 0%
Ln/P100/S10/D10-4     28.0ms ± 0%
Ln/P100/S10/D100-4    24.1ms ± 0%
Ln/P100/S100/D2-4     32.5ms ± 0%
Ln/P100/S100/D10-4    34.2ms ± 0%
Ln/P100/S100/D100-4   26.9ms ± 0%
```

cockroachdb/apd,mjibson/apd