ResultOr<T> not supported Implicit construction

Like `std::make_shared<T>()`, it support implicit construction.

such as, bad case:

```c++
auto f = []() -> ResultOr<std::shared_ptr<int>> {
    return nullptr;
};
```

good case:

```c++
auto f = []() -> ResultOr<std::shared_ptr<int>> {
    return std::shared_ptr<int>(nullptr);
};
```