rectangles: Use &[T] instead of &Vec<T> for signatures
rectangles: Use &[T] instead of &Vec<T> for signatures

Clippy suggested:
writing `&Vec<_>` instead of `&[_]` involves one more reference and
cannot be used with non-Vec-based slices.

https://rust-lang-nursery.github.io/rust-clippy/master/index.html#ptr_arg

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