Simplified setup_minimize_wrt().

The idea is mostly to get away from using indices in the context of
sets, which should help replacing the current set implementation with
another (B+ tree or red-black tree with efficient clones):

* [vector|set]_[const_]iter() now have no `from' argument
* vector_[const_]iter_from() has an index `from' argument
* set_[const_]iter_from() has an element `from' argument, whose
  (would-be) position is taken as starting point of the iterator
* setup_minimize_wrt() now takes a clause instead of a clause's index
* setup_minimize_wrt() doesn't use set_remove_index_range() anymore