derive Copy for trivial struct

[sineptic]

No description provided.

[asukaminato0721]

error: using `clone` on type `FSRSReview` which implements the `Copy` trait
  --> src/dataset.rs:50:9
   |
50 | /         self.reviews
51 | |             .iter()
52 | |             .find(|review| review.delta_t > 0)
53 | |             .unwrap()
54 | |             .clone()
   | |____________________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
   = note: `-D clippy::clone-on-copy` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::clone_on_copy)]`
help: try dereferencing it
   |
50 ~         *self.reviews
51 +             .iter()
52 +             .find(|review| review.delta_t > 0)
53 +             .unwrap()