Use inline ctype functions for ato*/strto* and scanf.

This also let us remove the `unsigned char` casts, since we define our
inlines to work for all values.

Before:

  Benchmark                      Time           CPU Iterations
  -------------------------------------------------------------
  BM_inttypes_strtoimax        112 ns        112 ns    6222193
  BM_inttypes_strtoumax        104 ns        104 ns    6725010
  BM_stdlib_strtol             113 ns        113 ns    6195861
  BM_stdlib_strtoll            113 ns        113 ns    6195633
  BM_stdlib_strtoul            105 ns        105 ns    6691394
  BM_stdlib_strtoull           105 ns        105 ns    6690695

  BM_stdio_scanf_d                    504 ns        503 ns    1385224
  BM_stdio_scanf_maps                1900 ns       1898 ns     369260
  BM_stdio_scanf_maps_baseline       1030 ns       1030 ns     678832
  BM_stdio_scanf_s                    433 ns        432 ns    1619086

After:

  BM_inttypes_strtoimax         91 ns         91 ns    7718194
  BM_inttypes_strtoumax         82 ns         82 ns    8508052
  BM_stdlib_strtol              92 ns         92 ns    7674694
  BM_stdlib_strtoll             91 ns         91 ns    7639228
  BM_stdlib_strtoul             83 ns         82 ns    8500304
  BM_stdlib_strtoull            82 ns         82 ns    8504929

  BM_stdio_scanf_d                    465 ns        465 ns    1507891
  BM_stdio_scanf_maps                1836 ns       1836 ns     381082
  BM_stdio_scanf_maps_baseline        846 ns        845 ns     830881
  BM_stdio_scanf_s                    419 ns        419 ns    1671979

Bug: N/A
Test: ran tests, benchmarks
Change-Id: I44681daf16c4328b060770cf11fc0633157c427f