Tabulators, reducers, etc

[Brian Goetz]

> One option might be: use "reduce" for the purely functional forms, use
> accumulate/accumulateConcurrent for the others:
>
>      T reduce(T zero, BinaryOperator<T> reducer);
>      Optional<T> reduce(BinaryOperator<T> reducer);
>      <U> U reduce(U zero, BiFunction<U, T, U> accumulator,
> BinaryOperator<U> reducer);
>
>      <R> R accumulate(Accumulator<T, R> reducer);
>      <R> R accumulate(Supplier<R> seedFactory,
>                       BiBlock<R, T> accumulator,
>                       BiBlock<R, R> reducer);
>
>      <R> R accumulateConcurrent(ConcurrentAccumulator<T, R> tabulator);
>
>    This would let us get rid of the Tabulator abstraction (it is
> identical to MutableReducer; both get renamed to Accumulator).
> Separately, with a small crowbar, we could simplify
> ConcurrentAccumulator down to fitting into existing SAMs, and the
> top-level abstraction could go away.

While the concurrent use case is clearly the odd man out here -- 
suggesting more work is left to do on this -- the rest of it seems an 
improvement on what we have now.  I would like to move forward with this 
while we continue to work out the correct set of canned accumulators and 
the correct way to surface concurrent accumulation.