Consider to add namespace-specific version of to_chars(...)

Consider adding a namespace-specific version of to_chars(...)

See also (track from, borrow from) corresponding [work in cnl](https://github.com/johnmcfarlane/cnl/issues/981)

```C
<namespace-specific>::to_chars_result <namespace-specific>::to_chars
  (
    char* first,
    char* last,
    value,
    int base = 10
  );
```