Support "unpacking" null to provide error-style handling of null values

[AlisCode]

Hi!

I'm evaluating using this package instead of maintaining my own Option[T] type.

One thing I've noticed is missing from this package is the possibility to unpack a null value into a tuple (T, bool).
Something like func (v *null.Value[T]) Unpack() (T, bool).

This does clone T which is fine for int/bool, maybe it's not fine for larger structs and it would be preferrable to return (*T, bool) ?

This allows to handle the case where the value is not initialized just like you would handle an error in idiomatic Go, i.e.

myValue, ok := nullableValue.Unpack()
if ok {
    // Do something with myValue, asserted to be non-null
}

I think the use-case is valid, any thoughts ?
This is not too hard to implement I think, I'm willing to contribute the change.

[guregu]

Personally I wouldn't use this library as a generic Option[T] type because it's too tied to SQL. It would work fine, but I think it would be better to use something else.

I am not opposed to adding an Unpack method, if people think it would be useful.
You can use the Ptr() method if you need a pointer to the value. It will be nil for null values, (*T, bool) is unnecessary IMO.