Implement vararg methods for mapreduce similar to map, fixes #27704.

[fredrikekre]

This is not really an optimal implementation, but I wanted to try to get the functionality in for 1.2. Should be fine to make improvements later.

[tkf]

I think this can be optimized by lowering to Broadcasted-based mapreduce in #31020 in the future. That is to say, mapreduce(f, op, x, y, z) can call whatever mapreduce(identity, op, Broadcast.instantiate(Broadcast.broadcasted(f, x, y, z)))) would call internally.

[fredrikekre]

bump

[StefanKarpinski]

@mbauman: do you think you could review so that this can make it into 1.2?

[mbauman]

LGTM! Thanks @fredrikekre.