Implement .zip() specialization for Map and Cloned.
The crucial thing for transparent specialization is that we want to
preserve the potential side effects.
The simplest example is that in this code snippet:
`(0..6).map(f).zip((0..4).map(g)).count()`
`f` will be called five times, and `g` four times. The last time for `f`
is when the other iterator is at its end, so this element is unused.
This side effect can be preserved without disturbing code generation for
simple uses of `.map()`.
The `Zip::next_back()` case is even more complicated, unfortunately.
