Do not parametrize abstract domain by the search strategy

[ptal]

Instead of relying on functor for the search strategy, pass it as an argument to split. For now, the argument is mostly useful to split over a subset of the variables, but should be extended to various strategies. An advantage of this design is that we could change dynamically the search strategy of an abstract element, or have two search strategies over distinct set of variables. This is possible in the current design but not convenient as dedicated module must be programmed by hand.