Specialize StepBy<Range<{integer}>>

For ranges < usize we determine the number of items
StepBy would yield and then store that in the range.end
instead of the actual end. This significantly
simplifies calculation of the loop induction variable
especially in cases where StepBy::step (an usize)
could overflow the Range's item type

library/core/benches/iter.rs

@@ -2,6 +2,7 @@ use core::borrow::Borrow;
 use core::iter::*;
 use core::mem;
 use core::num::Wrapping;
+use core::ops::Range;
 use test::{black_box, Bencher};
 
 #[bench]
@@ -69,6 +70,57 @@ fn bench_max(b: &mut Bencher) {
     })
 }
 
+#[bench]
+fn bench_range_step_by_sum_reducible(b: &mut Bencher) {
+    let r = 0u32..1024;
+    b.iter(|| {
+        let r = black_box(r.clone()).step_by(8);
+
+        let mut sum: u32 = 0;
+        for i in r {
+            sum += i;
+        }
+
+        sum
+    })
+}
+
+#[bench]
+fn bench_range_step_by_loop_u32(b: &mut Bencher) {
+    let r = 0..(u16::MAX as u32);
+    b.iter(|| {
+        let r = black_box(r.clone()).step_by(64);
+
+        let mut sum: u32 = 0;
+        for i in r {
+            let i = i ^ i.wrapping_sub(1);
+            sum = sum.wrapping_add(i);
+        }
+
+        sum
+    })
+}
+
+#[bench]
+fn bench_range_step_by_fold_usize(b: &mut Bencher) {
+    let r: Range<usize> = 0..(u16::MAX as usize);
+    b.iter(|| {
+        let r = black_box(r.clone());
+        r.step_by(64)
+            .map(|x: usize| x ^ (x.wrapping_sub(1)))
+            .fold(0usize, |acc, i| acc.wrapping_add(i))
+    })
+}
+
+#[bench]
+fn bench_range_step_by_fold_u16(b: &mut Bencher) {
+    let r: Range<u16> = 0..u16::MAX;
+    b.iter(|| {
+        let r = black_box(r.clone());
+        r.step_by(64).map(|x: u16| x ^ (x.wrapping_sub(1))).fold(0u16, |acc, i| acc.wrapping_add(i))
+    })
+}
+
 pub fn copy_zip(xs: &[u8], ys: &mut [u8]) {
     for (a, b) in ys.iter_mut().zip(xs) {
         *a = *b;