mappedTo -> join?

[Brian Goetz]

One of the reducers we've got takes a Stream<T> and a function T->U and 
creates a Map<T,U>.  Currently we call this "mappedTo".  Should this be 
called "join" instead?  Or mapJoin?

   Map<Person, Integer> map = people.reduce(mappedTo(Person::getAge)));
vs
   Map<Person, Integer> map = people.reduce(join(Person::getAge)));
   Map<Person, Integer> map = people.reduce(mapJoin(Person::getAge)));