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_feature/src/unstable.rs

@@ -1,11 +1,11 @@
 //! List of the unstable feature gates.
 
-use super::{to_nonzero, Feature};
-
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::Span;
 
+use super::{to_nonzero, Feature};
+
 pub struct UnstableFeature {
     pub feature: Feature,
     pub set_enabled: fn(&mut Features),