Review `FlatZipMap`

[kpreid]

I wrote exhaust::iteration::FlatZipMap as a helper to make it easier to implement certain patterns of iteration. I suspect that the Map part is obsolete now that factories exist, but I have not confirmed this.

[kpreid]

There is also the question of whether FlatZipMap should exist at all. The test of usefulness for what FlatZipMap was supposed to be is: can one write a nontrivial, non-derived implementation of Exhaust using an iterator that's primarily a composition of one or more FlatZipMaps?

The things that this can hopefully do that delegating to a derived implementation can't are:

• Fields with constraints between each other (i.e. not a pure cartesian product)
• Providing all the optional utilities like size_hint(), without needing to write a bunch of delegating methods.