Combine StreamSupport.stream/parallelStream?

[Brian Goetz]

A possible minor API cleanup:

For each pair of low-level methods stream() and parallelStream() in 
StreamSupport, we could combine these into a single method that takes an 
additional boolean isParallel argument.  This matches the underlying 
implementation they call anyway, and simplifies callers like concat, 
which now does:

         return (a.isParallel() || b.isParallel())
                ? StreamSupport.parallelStream(split)
                : StreamSupport.stream(split);

anyway.