Rollup merge of #55932 - Turbo87:to_digit, r=alexcrichton

core/char: Speed up `to_digit()` for `radix <= 10`

I noticed that `char::to_digit()` seemed to do a bit of extra work for handling `[a-zA-Z]` characters. Since `to_digit(10)` seems to be the most common case (at least in the `rust` codebase) I thought it might be valuable to create a fast path for that case, and according to the benchmarks that I added in one of the commits it seems to pay off. I also created another fast path for the `radix < 10` case, which also seems to have a positive effect.

It is very well possible that I'm measuring something entirely unrelated though, so please verify these numbers and let me know if I missed something!

### Before

```
# Run 1
test char::methods::bench_to_digit_radix_10      ... bench:      16,265 ns/iter (+/- 1,774)
test char::methods::bench_to_digit_radix_16      ... bench:      13,938 ns/iter (+/- 2,479)
test char::methods::bench_to_digit_radix_2       ... bench:      13,090 ns/iter (+/- 524)
test char::methods::bench_to_digit_radix_36      ... bench:      14,236 ns/iter (+/- 1,949)

# Run 2
test char::methods::bench_to_digit_radix_10      ... bench:      16,176 ns/iter (+/- 1,589)
test char::methods::bench_to_digit_radix_16      ... bench:      13,896 ns/iter (+/- 3,140)
test char::methods::bench_to_digit_radix_2       ... bench:      13,158 ns/iter (+/- 1,112)
test char::methods::bench_to_digit_radix_36      ... bench:      14,206 ns/iter (+/- 1,312)

# Run 3
test char::methods::bench_to_digit_radix_10      ... bench:      16,221 ns/iter (+/- 2,423)
test char::methods::bench_to_digit_radix_16      ... bench:      14,361 ns/iter (+/- 3,926)
test char::methods::bench_to_digit_radix_2       ... bench:      13,097 ns/iter (+/- 671)
test char::methods::bench_to_digit_radix_36      ... bench:      14,388 ns/iter (+/- 1,068)
```

### After

```
# Run 1
test char::methods::bench_to_digit_radix_10      ... bench:      11,521 ns/iter (+/- 552)
test char::methods::bench_to_digit_radix_16      ... bench:      12,926 ns/iter (+/- 684)
test char::methods::bench_to_digit_radix_2       ... bench:      11,266 ns/iter (+/- 1,085)
test char::methods::bench_to_digit_radix_36      ... bench:      14,213 ns/iter (+/- 614)

# Run 2
test char::methods::bench_to_digit_radix_10      ... bench:      11,424 ns/iter (+/- 1,042)
test char::methods::bench_to_digit_radix_16      ... bench:      12,854 ns/iter (+/- 1,193)
test char::methods::bench_to_digit_radix_2       ... bench:      11,193 ns/iter (+/- 716)
test char::methods::bench_to_digit_radix_36      ... bench:      14,249 ns/iter (+/- 3,514)

# Run 3
test char::methods::bench_to_digit_radix_10      ... bench:      11,469 ns/iter (+/- 685)
test char::methods::bench_to_digit_radix_16      ... bench:      12,852 ns/iter (+/- 568)
test char::methods::bench_to_digit_radix_2       ... bench:      11,275 ns/iter (+/- 1,356)
test char::methods::bench_to_digit_radix_36      ... bench:      14,188 ns/iter (+/- 1,501)
```

I ran the benchmark using:

```sh
python x.py bench src/libcore --stage 1 --keep-stage 0 --test-args "bench_to_digit"
```

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