Initial spec review for Stream

[Brian Goetz]

I've put up some very rough proto-spec for Stream and the stream 
package-info at: 
http://cr.openjdk.java.net/~briangoetz/JDK-8008682/doc/.  (I've included 
the whole package but am only requesting comments on these two files for 
now, as the rest are incomplete.)  There's definitely lots of stuff 
missing, including:

  - Describe the difference between sequential and parallel streams
  - More general information about reduce, better definitions for 
associativity, more description of how reduce employs parallelism, more 
examples
  - Role of stream flags in various operations, specifically ordering
  - Non-interference and constraints on lambda characteristics (e.g., 
side-effect-freedom)
  - collectUnordered

But its a start.  Comments please!