The previous API was not flexible enough to implement the new
`AttributeManager` with all "bells and whistles" we want.
For example, the new `addImportedFn` field allows us to initialize
the state for the imported entries using a `Thunk`.
It seems `lazy := false` is only going to be used in the attribute
manager. So, I remove it. I added a new field `addImported : Bool`
instead. An extension can specify whether `addEntryFn` is going to be
invoked or not for imported entries. `addImported := false` is useful for extensions such
as `protected`, and I will use it in the attribute manager too.
