Collectors inventory

[Brian Goetz]

As I promised a long time ago, here's an overview of what's in 
Collectors currently.

There are 12 basic forms:
  - toCollection(ctor)
  - toList()
  - toSet()
  - toStringBuilder()
  - toStringJoiner(delimiter)
  - to{Long,Double}Statistics

  - groupingBy(classifier, mapFactory, downstream collector)
  - groupingReduce(classifier, mapFactory, mapper, reducer)
  - mapping(mappingFn, downstream collector)
  - joiningWith(mappingFunction, mergeFunction, mapFactory)
  - partitioningBy(predicate, downstream collector)
  - partitioningReduce(predicate, mapper, reducer)

The toXxx forms should be obvious.

Mapping has four versions, analogous to Stream.map:
  - mapping(T -> U, Collector<U, R>)
  - mapping(T -> int, Collector.OfInt<R>)
  - mapping(T -> long, Collector.OfLong<R>)
  - mapping(T -> double, Collector.OfDouble<R>)

GroupingBy has four forms:
  - groupingBy(T->K) -- standard groupBy, values of resulting Map are 
Collection<T>
  - Same, but with explicit constructors for map and for rows (so you 
can produce, say, a TreeMap<K, TreeSet<T>> and not just a 
Map<K,Collection<T>>)
  - groupingBy(T->K, Collector<T,D>) -- multi-level groupBy, where 
downstream is another Collector
  - Same, but with explicit ctor for map

GroupingReduce has four forms:
  - groupingReduce(T->K, BinaryOperator<T>) // simple reduce
  - groupingReduce(T->K, Function<T,U>, BinaryOperator<U>) // map-reduce
  - above two with explicit map ctors

JoiningWith has four forms:
  - joiningWith(T->U)
  - same, but with explicit Map ctor
  - same, but with merge function for handling duplicates
  - same, with both explicit map ctor and merge function

PartitioningBy has three forms:
  - partitioningBy(Predicate)
  - Same, but with explicit constructor for Collection (so you can get a 
Map<Boolean, TreeSet<T>>)
  - partitioningBy(Predicate, Collector) // multi-level

PartitioningReduce has two forms:
  - predicate + reducer
  - predicate + mapper + reducer

Impl note: in any category, all but one are one-liners that delegate to 
the general form.

Plus, all the Map-bearing ones have a concurrent and non-concurrent 
version.