Merge #19027

19027: sys/fmt: optimize scn_u32_dec scn_u32_hex r=benpicco a=kfessel

### Contribution description

Improves the compilation result for `scn_u32_dec` `scn_u32_hex` especially on `cortex-m` reducing either stack usage and or code size.

This makes use of unsigned int overflow (slightly less better without doing that `hexn`).

See godbolt (original versions got an `o` attached, modified versions got `k`s) all functions are  marked `_S_` defined to `static`

by assigning the global at end the compiled function can be changed (`deco deck  hexo hexk hexkk hexn`)

this PR is `hexkk` and `deck` 

### Testing procedure

run unit-test/test-fmt

```
<RIOT>/tests/unittests$ make tests-fmt
<RIOT>/tests/unittests$ make term
```

### Issues/PRs references

[godbolt](https://godbolt.org/z/MzT1zh4q1)

Co-authored-by: Karl Fessel <karl.fessel@ovgu.de>

sys/fmt/fmt.c

@@ -471,14 +471,13 @@ uint32_t scn_u32_dec(const char *str, size_t n)
  uint32_t res = 0;
 
  while (n--) {
- char c = *str++;
- if (!fmt_is_digit(c)) {
+ unsigned c = *str++;
+ unsigned d = c - (unsigned)'0';
+ if ( !( '0'<= c && c <= '9') ) {
  break;
  }
- else {
- res *= 10;
- res += (c - '0');
- }
+ res *= 10U;
+ res += d;
  }
  return res;
 }
@@ -488,21 +487,23 @@ uint32_t scn_u32_hex(const char *str, size_t n)
  uint32_t res = 0;
 
  while (n--) {
- char c = *str++;
- if (!fmt_is_digit(c)) {
-  if (fmt_is_upper(c)) {
-  c = _to_lower(c);
-  }
-  if (c == '\0' || c > 'f') {
-  break;
-  }
-  res <<= 4;
- res |= c - 'a' + 0xa;
+ unsigned c = *str++;
+ unsigned d;
+ if (('0'<= c) && (c <= '9')){
+ d = c - (unsigned)'0';
+ }
+ else if (('A' <= c) && (c <= 'F')) {
+ d = c - (unsigned)'A' + 0xaU;
+ }
+ else if (('a' <= c) && (c <= 'f')) {
+ d = c - (unsigned)'a' + 0xaU;
  }
  else {
- res <<= 4;
- res |= c - '0';
+ break;
  }
+ res <<= 4U;
+ res |= d;
+
  }
  return res;
 }