Add an explicit Box<T>.DangerousSetValue method to update the boxed value without allocating a new instance

[AlexRadch]

Sometimes it is useful to change a boxed value without creating a new instance.
This can be done implicitly using the GetReference method boxed.GetReference() = newValue;

Would it be useful to create an explicit method for this? boxed.DangerousSetValue(newValue);

The method name may be different: DangerousSetValue, DangerousUpdateValue, DangerousChangeValue etc.