Add `Iterator::for_each`

This works like a `for` loop in functional style, applying a closure to
every item in the `Iterator`.  It doesn't allow `break`/`continue` like
a `for` loop, nor any other control flow outside the closure, but it may
be a more legible style for tying up the end of a long iterator chain.

This was tried before in #14911, but nobody made the case for using it
with longer iterators.  There was also `Iterator::advance` at that time
which was more capable than `for_each`, but that no longer exists.

The `itertools` crate has `Itertools::foreach` with the same behavior,
but thankfully the names won't collide.  The `rayon` crate also has a
`ParallelIterator::for_each` where simple `for` loops aren't possible.

> I really wish we had `for_each` on seq iterators. Having to use a
> dummy operation is annoying.  - [@nikomatsakis][1]

[1]: https://github.com/nikomatsakis/rayon/pull/367#issuecomment-308455185

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