Constructing parallel streams

[Brian Goetz]

> I don't like users being able to call parallel in the middle of the
> stream construction.

I don't love it either.  The semantics are perfectly tractible, and the 
implementation is perfectly straightforward, but the performance is 
unlikely to be a win in most cases.  (I mentioned earlier we would doc 
that this really should only be done at the head of the pipeline.)

> I propose to have an interface ParallelizableStream that allows to
> choose if the user want the sequential or the parallel stream upfront.

Yeah, we investigated this direction first.  Combinatorial explosion: 
IntParallelizableStream, etc.

However, this could trivially become a dynamic property of streams (fits 
easily into the existing stream flags mechanism).  Then only the head 
streams would have the property, and if you tried to do parallel() 
farther down the stream, we could ignore it or even throw ISE.