Improve the implementation of itertools.tee().

Formerly, underlying queue was implemented in terms of two lists.  The
new queue is a series of singly-linked fixed length lists.

The new implementation runs much faster, supports multi-way tees, and
allows tees of tees without additional memory costs.

The root ideas for this structure were contributed by Andrew Koenig
and Guido van Rossum.

Doc/lib/libitertools.tex

@@ -281,9 +281,9 @@ by functions or loops that truncate the stream.
   \end{verbatim}
 \end{funcdesc}
 
-\begin{funcdesc}{tee}{iterable}
-  Return two independent iterators from a single iterable.
-  Equivalent to:
+\begin{funcdesc}{tee}{iterable\optional{, n=2}}
+  Return \var{n} independent iterators from a single iterable.
+  The case where \var{n} is two is equivalent to:
 
   \begin{verbatim}
      def tee(iterable):
@@ -299,6 +299,10 @@ by functions or loops that truncate the stream.
          return (gen(it.next), gen(it.next))
   \end{verbatim}
 
+  Note, once \function{tee()} has made a split, the original \var{iterable}
+  should not be used anywhere else; otherwise, the \var{iterable} could get
+  advanced without the tee objects being informed.
+
   Note, this member of the toolkit may require significant auxiliary
   storage (depending on how much temporary data needs to be stored).
   In general, if one iterator is going use most or all of the data before
@@ -408,6 +412,10 @@ def repeatfunc(func, times=None, *args):
 def pairwise(iterable):
     "s -> (s0,s1), (s1,s2), (s2, s3), ..."
     a, b = tee(iterable)
-    return izip(a, islice(b, 1, None))
+    try:
+        b.next()
+    except StopIteration:
+        pass
+    return izip(a, b)
 
 \end{verbatim}