Use `fold` to implement `Iterator::for_each`

The benefit of using internal iteration is shown in new benchmarks:

    test iter::bench_for_each_chain_fold     ... bench:     635,110 ns/iter (+/- 5,135)
    test iter::bench_for_each_chain_loop     ... bench:   2,249,983 ns/iter (+/- 42,001)
    test iter::bench_for_each_chain_ref_fold ... bench:   2,248,061 ns/iter (+/- 51,940)

src/libcore/benches/iter.rs

@@ -99,3 +99,50 @@ fn bench_zip_add(b: &mut Bencher) {
        add_zip(&source, &mut dst)
    });
}

/// `Iterator::for_each` implemented as a plain loop.
fn for_each_loop<I, F>(iter: I, mut f: F) where
    I: Iterator, F: FnMut(I::Item)
{
    for item in iter {
        f(item);
    }
}

/// `Iterator::for_each` implemented with `fold` for internal iteration.
/// (except when `by_ref()` effectively disables that optimization.)
fn for_each_fold<I, F>(iter: I, mut f: F) where
    I: Iterator, F: FnMut(I::Item)
{
    iter.fold((), move |(), item| f(item));
}

#[bench]
fn bench_for_each_chain_loop(b: &mut Bencher) {
    b.iter(|| {
        let mut acc = 0;
        let iter = (0i64..1000000).chain(0..1000000).map(black_box);
        for_each_loop(iter, |x| acc += x);
        acc
    });
}

#[bench]
fn bench_for_each_chain_fold(b: &mut Bencher) {
    b.iter(|| {
        let mut acc = 0;
        let iter = (0i64..1000000).chain(0..1000000).map(black_box);
        for_each_fold(iter, |x| acc += x);
        acc
    });
}

#[bench]
fn bench_for_each_chain_ref_fold(b: &mut Bencher) {
    b.iter(|| {
        let mut acc = 0;
        let mut iter = (0i64..1000000).chain(0..1000000).map(black_box);
        for_each_fold(iter.by_ref(), |x| acc += x);
        acc
    });
}

src/libcore/iter/iterator.rs

@@ -487,7 +487,9 @@ pub trait Iterator {
    /// This is equivalent to using a [`for`] loop on the iterator, although
    /// `break` and `continue` are not possible from a closure.  It's generally
    /// more idiomatic to use a `for` loop, but `for_each` may be more legible
    /// when processing items at the end of longer iterator chains.
    /// when processing items at the end of longer iterator chains.  In some
    /// cases `for_each` may also be faster than a loop, because it will use
    /// internal iteration on adaptors like `Chain`.
    ///
    /// [`for`]: ../../book/first-edition/loops.html#for
    ///
@@ -523,9 +525,7 @@ pub trait Iterator {
    fn for_each<F>(self, mut f: F) where
        Self: Sized, F: FnMut(Self::Item),
    {
        for item in self {
            f(item);
        }
        self.fold((), move |(), item| f(item));
    }

    /// Creates an iterator which uses a closure to determine if an element