Concat and zip

[Brian Goetz]

We're almost finished syncing the Streams code from the lambda sandbox 
into the jdk8 repositories.  One of the last items on the list are the 
"concat" and "zip" methods we've currently got in "Streams".

First, these aren't "done" because we only have reference versions, not 
Int/Long/Double versions.  So more work would be required to finish 
these, or we'd have to take the hit for only having reference versions 
(and endless requests to add specializations.  And we're also missing 
the SAMs that would be needed for primitive versions of zip.)

Second, I'm starting to have YAGNI thoughts on these, especially zip. 
Zip shows up all over the place in functional languages, but the 
efficacy of these idioms relies on tuples being cheap.  Also zip 
parallelizes basically not at all.