Rework ActiveValue

[tyt2y3]

Currently our ActiveValue looks like this:

pub struct ActiveValue<V>
where
    V: Into<Value>,
{
    value: Option<V>,
    state: ActiveValueState,
}

I am thinking to go back to a simple Enum:

pub enum ActiveValue<V>
where
    V: Into<Value>,
{
    Set(V),
    Unchanged(V),
    NotSet,
}

The overall API should remain the same, just that we should deprecate Unset(None) and favour NotSet instead.