Define mapreduce with multiple iterators for operations over both values and indices

I went down the rabbit hole of trying to implement argmin / findmin using AK primitives without scalar indexing.

Essentially, the solution would be this:
https://github.com/JuliaGPU/GPUArrays.jl/blob/master/src/host/indexing.jl#L229

Which requires `mapreduce(f, op, itr...)` combined with `tuple` to create each tuple `(idx, val)`.