varied arguments to solvers and scorers should result in distinct `task_id`s

[simonpcouch]

The new version of the vendored log viewer (#203) collapses tasks with the same task_id as "Retried" logs. We currently only hash based on what's available at Task$new(), but I think we'll instead need to do so at Task$log() using information about the solver and scorer. Let's align more closely with Inspect here.