`ConflictResolver::resolve_conflict` should receive `Conflict`

[maartenflippo]

At the moment, the ConflictResolutionContext gets the StoredConflictInfo from the solver state. However, it contains two variants that can never be true if the resolver is called correctly. Namely, InconsistentAssumptions and RootLevelConflict. Semantically it is clearer if we pass the Conflict that is produced by State::propagate_to_fixed_point to the resolver directly.