Consider enabling CTAD for the safe type

CTAD: class template argument deduction

Such thing will allow this use case, in C++20 and higher.

```cpp
#include

void uncontrolled(double*, int); // true external function

using boost::safe_numerics::safe;

int main() {

std::vector v = {1.0, 2.0, 3.0};

uncontrolled(v.data(), safe{v.max_size()}); // ok, throws

// uncontrolled(v.data(), safe{v.max_size()}); // needs CTAD
}
```

complete code: https://godbolt.org/z/s8bYz9rqb

The most basic code for this feature could be:

```cpp
#if define(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201703L)
namespace boost::safe_numerics {
template safe_base(In) -> safe_base;
}
#endif
```

Start with boost/safe_numerics/safe_integer.hpp and the safe type used in the provided C++20 example. Check how the existing safe template is declared and how the proposed deduction guide would apply. Done means the safe{v.max_size()} form compiles and provides the expected safe integer type under the supported deduction-guide feature test.

Written by the indexing model from the issue text.