Implement `Valuable` for Option<T> and `Result<T>` about valuable

[taiki-e]

I completely agree with you about the Option.

However, I'm not sure about the Result: how do we handle the Err variant if the std feature is disabled?

[hawkw]

I agree that Option's should almost certainly flatten the value. I'm less convinced about Result --- if the Err side of a Result is not a &(dyn std::error::Err + 'static), there's no way for the visitor to know if the value is an Ok or an Err.

As a (somewhat contrived) example, if I recorded a Result<usize, usize>, the visitor would totally lose the information of whether the Result was an Ok or an Err...which may actually be the only information that we actually want to record.

IMO, Results should be Enumerable, so that the information of whether it was Ok or Err is always retained. But, I could probably be convinced otherwise by a sufficiently good argument...