Collectors inventory

[Brian Goetz]

After working through the Collectors, here's the current inventory, 
which is looking pretty good:

   - Collections
     - toCollection(supplier)
     - toList()
     - toSet()
     - toMap x3: keyMapper, valueMapper [, mergeFn [, mapSupplier ] ]
     - toConcurrentMap x3

   - Grouping combinators
     - groupingBy x3: classifier [, downstream [, mapSupplier ] ]
     - groupingByConcurrent x3
     - partitioningBy x2: predicate [, mapSupplier ]

   - Mapping combinators
     - mapping(f, downstream)

   - String-specific reductions
     - concatenating()
     - joining x2: delimiter [, prefix, suffix ]

   - Reductions:
     - reducing x3: reducer [, identity [, mapper ] ]
     - counting()
     - minBy(comparator)
     - maxBy(comparator)
     - summing{Int,Long,Double}(extractorFn)
     - averaging{Int,Long,Double}(extractorFn)
     - summarizing{Int,Long,Double}(extractorFn)