feat: derive Hash for FSRSReview and FSRSItem

[sineptic]

No description provided.

[L-M-Sherlock]

Thank you for your contribution! Could you please provide more details on why you decided to derive the Hash trait for FSRSReview and FSRSItem? Understanding the motivation behind this change will help us in the review process.

[sineptic]

Excuse me please. I feeling so guilty, I don't think before trying to contribute.

My task manager type has tasks_to_complete and tasks_pool. I wrote function find_tasks_to_recall, that (I think) find tasks to recall in fastest way, but only approach for this is implement Cmp(compare time to recall) for tasks wrapper(to use in BTreeSet and stop when first not to complete found). I quickly realize, that comparing time with default parameters not work for me, so BTreeSet -> HashSet and scan all for tasks to complete, but I don't think that Vec could replace HashSet