Fixed potential bug with -MINIMUM_VALUE

Fixed potential bug with -MINIMUM_VALUE

Original code had something like `N < 0 ? -N : N` which could fail if `N` has minimum value of appropriate type. For example, in original code this could fail:
```cpp
const long long int LL_MIN = std::numeric_limits<long long int>::min(); //probably: -9223372036854775808
static const char *number = to_string<LL_MIN, 10>;
```

Also note that this code relies on "modulo on negative first operands" which was implementation defined in C++03 and before, but with C++11 and later it is defined by standard (see `ISO14882:2011(e)`), and this code anyway targets C++17 so we're safe here.