Multi-argument `eachindex`

[lindahua]

The eachindex is a great function, which provides a uniform & efficient way to traverse a array.

However, in many cases, the computation require multiple arguments, e.g., addition.

s = rand(5, 6)
a = rand(4, 6)   # with fast linear indexing
b = sub(s, 1:4, :)   # no fast linear indexing
c = zeros(4, 6)

# It would be great to do the following
for (ia, ib, ic) in eachindex(a, b, c)
    c[ic] = a[ia] + b[ib]
end

A more greedy demand is to allow broadcasting eachindex when arrays have different sizes, but their shapes are compatible (in terms of broadcasting).

cc: @timholy

[timholy]

Agreed on both points. The two-argument version was just implemented in #10858, but I agree we could probably use a multi-argument version. For the broadcasting idea, there's something like that in reducedim, but I think it only adds trailing 1s. You can use max(I1,I2) on two CartesianIndexes. I bet all this is not as flexible or easy to use as it should be, however.

[tkelman]

doesn't seem release-blocking