Move OrSuggestions to its own header

Prevents a recursive inclusion

src/libutil/suggestions.hh

@@ -39,63 +39,4 @@ public:
 
     Suggestions& operator+=(const Suggestions & other);
 };
-
-// Either a value of type `T`, or some suggestions
-template<typename T>
-class OrSuggestions {
-public:
-    using Raw = std::variant<T, Suggestions>;
-
-    Raw raw;
-
-    T* operator ->()
-    {
-        return &**this;
-    }
-
-    T& operator *()
-    {
-        if (auto elt = std::get_if<T>(&raw))
-            return *elt;
-        throw Error("Invalid access to a failed value");
-    }
-
-    operator bool() const noexcept
-    {
-        return std::holds_alternative<T>(raw);
-    }
-
-    OrSuggestions(T t)
-        : raw(t)
-    {
-    }
-
-    OrSuggestions()
-        : raw(Suggestions{})
-    {
-    }
-
-    static OrSuggestions<T> failed(const Suggestions & s)
-    {
-        auto res = OrSuggestions<T>();
-        res.raw = s;
-        return res;
-    }
-
-    static OrSuggestions<T> failed()
-    {
-        return OrSuggestions<T>::failed(Suggestions{});
-    }
-
-    const Suggestions & get_suggestions()
-    {
-        static Suggestions noSuggestions;
-        if (const auto & suggestions = std::get_if<Suggestions>(&raw))
-            return *suggestions;
-        else
-            return noSuggestions;
-    }
-
-};
-
 }