Fix signed/unsigned mismatch warnings, part 4.
Fix signed/unsigned mismatch warnings, part 4.

log10Pow2() and log10Pow5() are always used to initialize uint32_t:

```
const uint32_t q = log10Pow2(e2) - (e2 > 3);
const uint32_t q = log10Pow2(e2);
const uint32_t q = log10Pow5(-e2) - (-e2 > 1);
const uint32_t q = log10Pow5(-e2);
```

We can avoid compiler warnings by changing their return types
and statements accordingly, which removes two casts.

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu