Minor fix in the *_expensive benchmark
Minor fix in the *_expensive benchmark

Before, the `count` would be copied into the closure and could
potentially be optimized way. This change ensures it's borrowed by
closure and finally consumed by `test::black_box`.

graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust