Collectors inventory

[Brian Goetz]

Breaking this into multiple messages.  

> 2. Rename joinWith to toMap

Why didn't I think of this!  I like this.  Much clearer.

We currently have four forms, which form the cross product of:

 { with merge function, without } x { with explicit ctor, without }

If it makes the "too many Collectors" contingent happier, we can reduce this to two forms:

  <U> Collector<T, Map<T, U>> toMap(mapper)
  <U> Collector<T, Map<T, U>> toMap(mapper, merger, mapCtor)