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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