boost::safe_numerics::safe<...> is convertible to std::string?

Is it intentional that the following code does not produce an error when compiled?

```
#include
#include
#include

static_assert(std::is_convertible_v, std::string>);
```

It seems wrong, but maybe there is a reason for it?

I'm trying to track down a (to me) relatively obscure problem elsewhere, and I think this may be at least related to, if not the cause of the problems I'm seeing. I checked the two most recent versions of clang and gcc and got the same behavior in all cases.

The reproducer includes boost/safe_numerics/safe_integer.hpp and checks std::is_convertible_v, std::string>; start by compiling that static assertion with the reported compilers. Trace the safe conversion path and compare the observed result with the intended API behavior; done means the conversion behavior has a documented resolution and a regression check.

Written by the indexing model from the issue text.