Refactor `Legacy_Lifter`, make it optional

* Rename `Legacy_Lifter` to `Legacy_Lifter_Wrapper`

* Rename `Project_Legacy_Lifter` to `Legacy_Lifter`

* Factor `lift_change` and `lift_entity` from `Legacy_Lifter_Wrapper` to
  `Legacy_Lifter`

* Only use `Legacy_Lifter` if explicitly specified

  + Use `_MOM.Legacy_Lifter` to force default `legacy_lifter`.

    For instance::

        python model.py migrate \
            -Active -legacy_lifter _MOM.Legacy_Lifter

* Add cache for accesses to `E_Type_Lifter`