Concat operation

[Brian Goetz]

> Shouldn't it be illegal to concat on an infinite stream? I propose throwing
> ISE if the target stream is infinite.

Good point, though its not clear how much value this has.  There are 
some streams we know to be finite (e.g., those backed by an array or 
collection), some streams we know to be infinite (e.g., those backed by 
a generator), and *a lot* where we don't know either, such as:

   anyInfiniteStream.filter(lambda)
   anyFiniteStream.flatMap(lambda)

So while this is something we can check, checking it might only lead to 
a false sense of confidence.