htools: add ChangeGroup to IAllocator types/loader

This patch updates the types and functions in IAllocator.hs and
Loader.hs to the new design changes (elimination of multi-relocate
mode and replacement with change-group).

It also removes an extra re-export of EvacMode from Loader.hs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>