Add groupByEachUniqueKey to RichIterable

[donraab]

RichIterable currently has groupBy / groupByEach but only has groupByUniqueKey without the corresponding groupByEachUniqueKey. The method groupByEachUniqueKey would index an item in a Map using multiple keys. Each key will only point to one value. An example of this might be mapping a Person to his/her aliases.

I ran into this problem recently trying to group an Object that contains a distinct range of numbers by each of the distinct numbers. I was able to accomplish this by using a nested injectInto, but it is somewhat awkward and may be difficult to comprehend. In my particular case, it is more optimal using injectInto as I am grouping into an IntObjectMap.