Reversing order of `prod(itr)`

[stevengj]

@jlbosse

Note that in my case using Iterators.reverse does not work, because I get MethodError: no method matching iterate(::Base.Iterators.Reverse{Dict{Int64,String}}

If you are using a dictionary, the iteration order is undefined and may change from release to release. So I think you need a different data structure if you want a specific order.