grade-school: Use `to_vec` instead of `cloned().collect()`
grade-school: Use `to_vec` instead of `cloned().collect()`

Clippy suggested:
https://travis-ci.org/exercism/rust/builds/456356560
warning: called `cloned().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and more readable
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#iter_cloned_collect

petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust