More refactoring.

* tentative to make CRDT.Set having the signature Set.S

* this works, but then cast between different actors do not work (yet)
  and thus I had to use uglies Obj.magic in the test (shame on
  me). I'll improve that later.

I've started to implement Maps as well, but it needs a litle bit more
thoughts. Same for optimizing the distributed objects, need to be
carreful when deciding to clean-up some structures (currently, they
are ever-growing, so not suitable to use in production).