Extending Collector to handle a post-transform

[Brian Goetz]

> +1 on removing first/last wins mergers.
>
> I am pausing a little on the throwingMerger. We capture the most
> important case of throw-based merging in the 2 arg to{Concurrent}Map
> function, but what if someone wants to switch from the 2 arg to the 4
> arg method to change the map impl?
>
> Perhaps we can move throwingMerger to Map renamed to match the
> wording on Map.merge ?

Yes, I think that's a better place for mergers.