Eisel-Lemire is a fast Atod algorithm, discussed at
https://nigeltao.github.io/blog/2020/eisel-lemire.html

Median of 5 runs of "time atod_manual_test pnftd/data/*.txt"
user    0m1.477s  # Before
user    0m0.746s  # After (a speed-up of 1.98x)
where pnftd is https://github.com/nigeltao/parse-number-fxx-test-data

"benchy --reference=srcfs --benchmark_filter='SimpleAtod' :numbers_benchmark"
output (which uses deterministic but randomly generated input strings):
name                                    old cpu/op  new cpu/op  delta
BM_SimpleAtod<absl::string_view>/10/1   388ns ± 3%   330ns ± 2%  -15.06%  (p=0.000 n=49+44)
BM_SimpleAtod<absl::string_view>/10/2   428ns ± 3%   320ns ± 2%  -25.07%  (p=0.000 n=59+49)
BM_SimpleAtod<absl::string_view>/10/4   431ns ± 3%   353ns ± 4%  -18.04%  (p=0.000 n=59+48)
BM_SimpleAtod<absl::string_view>/10/8   512ns ± 3%   409ns ± 4%  -20.18%  (p=0.000 n=60+56)
BM_SimpleAtod<const char*>/10/1         411ns ± 4%   341ns ± 3%  -17.15%  (p=0.000 n=51+49)
BM_SimpleAtod<const char*>/10/2         431ns ± 3%   356ns ± 2%  -17.44%  (p=0.000 n=58+48)
BM_SimpleAtod<const char*>/10/4         437ns ± 3%   378ns ± 4%  -13.49%  (p=0.000 n=60+50)
BM_SimpleAtod<const char*>/10/8         552ns ± 2%   450ns ± 4%  -18.43%  (p=0.000 n=59+59)
BM_SimpleAtod<std::string>/10/1         404ns ± 2%   328ns ± 2%  -18.78%  (p=0.000 n=48+49)
BM_SimpleAtod<std::string>/10/2         438ns ± 3%   321ns ± 3%  -26.81%  (p=0.000 n=60+50)
BM_SimpleAtod<std::string>/10/4         446ns ± 2%   354ns ± 3%  -20.66%  (p=0.000 n=59+50)
BM_SimpleAtod<std::string>/10/8         519ns ± 2%   410ns ± 4%  -21.07%  (p=0.000 n=59+50)

sizeof(kPower10MantissaTable) doubles from 5208 to 10416 bytes.

PiperOrigin-RevId: 474174891
Change-Id: Iea6f6569419b6b55ab5978994f0adb1f4a8aafb2