Make optional_reference bool conversion explicit

If this is non-explicit, then the following code:

optional_reference<config> ref = ...;
auto str = ref["key"]

results in ref being converted to bool and used as an index into the key string.

src/utils/optional_reference.hpp

@@ -70,7 +70,7 @@ public:
 		return *this;
 	}
 
-	operator bool() const
+	explicit operator bool() const
 	{
 		return opt_.has_value();
 	}