Reverted for-loop, it messes up type inference

Author: llogiq

src/libtest/stats.rs

@@ -168,7 +168,8 @@ impl Stats for [f64] {
             let mut j = 0;
             // This inner loop applies `hi`/`lo` summation to each
             // partial so that the list of partial sums remains exact.
-            for mut y in &mut partials {
+            for i in 0..partials.len() {
+                let mut y: f64 = partials[i];
                 if x.abs() < y.abs() {
                     mem::swap(&mut x, &mut y);
                 }