Generic function for collect, fix clauses clauses combination

- Hashtbl stores a bool to remember if bhv has assumes clauses
-  When looking for clauses to combine for default behavior, we search
   in that order (and stop if we found some clauses) :
   - unguarded behaviors (no assumes).
   - complete behaviors (each one of them needs to be in our table).
   - all behaviors in our table.
     It is a generic function used by {!Generator.get_clauses}.