Removed `std::atoi` from the benchmarks since it was performing so poorly; not a contender. Should be easy to verify.

Rough results (last column is #iterations):

  BM_fast_int<std::int64_t>/10                  1961 ns         1958 ns       355081
  BM_fast_int<std::int64_t>/100                 2973 ns         2969 ns       233953
  BM_fast_int<std::int64_t>/1000                3636 ns         3631 ns       186585
  BM_fast_int<std::int64_t>/10000               4314 ns         4309 ns       161831
  BM_fast_int<std::int64_t>/100000              5184 ns         5179 ns       136308
  BM_fast_int<std::int64_t>/1000000             5867 ns         5859 ns       119398
  BM_fast_int_swar<std::int64_t>/10             2235 ns         2232 ns       316949
  BM_fast_int_swar<std::int64_t>/100            3446 ns         3441 ns       206437
  BM_fast_int_swar<std::int64_t>/1000           3561 ns         3556 ns       197795
  BM_fast_int_swar<std::int64_t>/10000          3650 ns         3646 ns       188613
  BM_fast_int_swar<std::int64_t>/100000         4248 ns         4243 ns       165313
  BM_fast_int_swar<std::int64_t>/1000000        4979 ns         4973 ns       140722
  BM_atoi<std::int64_t>/10                     10248 ns        10234 ns        69021
  BM_atoi<std::int64_t>/100                    10996 ns        10985 ns        63810
  BM_atoi<std::int64_t>/1000                   12238 ns        12225 ns        56556
  BM_atoi<std::int64_t>/10000                  13606 ns        13589 ns        51645
  BM_atoi<std::int64_t>/100000                 14984 ns        14964 ns        47046
  BM_atoi<std::int64_t>/1000000                16226 ns        16206 ns        43279
  BM_from_chars<std::int64_t>/10                2162 ns         2160 ns       302880
  BM_from_chars<std::int64_t>/100               2410 ns         2407 ns       282778
  BM_from_chars<std::int64_t>/1000              3309 ns         3306 ns       208070
  BM_from_chars<std::int64_t>/10000             5034 ns         5028 ns       100000
  BM_from_chars<std::int64_t>/100000            6282 ns         6275 ns       107023
  BM_from_chars<std::int64_t>/1000000           7267 ns         7259 ns        96114
  BM_fast_float<std::int64_t>/10                2670 ns         2666 ns       262721
  BM_fast_float<std::int64_t>/100               3547 ns         3542 ns       196704
  BM_fast_float<std::int64_t>/1000              4643 ns         4638 ns       154391
  BM_fast_float<std::int64_t>/10000             5056 ns         5050 ns       132722
  BM_fast_float<std::int64_t>/100000            6207 ns         6200 ns       111565
  BM_fast_float<std::int64_t>/1000000           7113 ns         7105 ns        98847