overload's constructor isn't constrained

Short example:

```
#include "boost/hana.hpp"

void f();
void g(int);

int main() {
auto a = boost::hana::overload(f,g);
auto b(a); // error
}
```

This is because `overload_t`'s constructor doesn't have any constraints on its first parameter (need to do the `!is_same_v, overload_t>` dance).

Locate the definition of overload_t and its constructor, then inspect the existing tests for overload behavior. Reproduce the copy-initialization example and verify that constructing an overload_t from another overload_t no longer selects the unconstrained constructor, with the relevant regression test passing.

Written by the indexing model from the issue text.