Reformat use declarations.

The previous commit updated `rustfmt.toml` appropriately. This commit is
the outcome of running `x fmt --all` with the new formatting options.

compiler/rustc_index/src/bit_set/tests.rs

@@ -2,6 +2,7 @@ use super::*;
 
 extern crate test;
 use std::hint::black_box;
+
 use test::Bencher;
 
 #[test]