Generalize dictionary() to accept a sequence of 2-sequences. At the

outer level, the iterator protocol is used for memory-efficiency (the
outer sequence may be very large if fully materialized); at the inner
level, PySequence_Fast() is used for time-efficiency (these should
always be sequences of length 2).

dictobject.c, new functions PyDict_{Merge,Update}FromSeq2.  These are
wholly analogous to PyDict_{Merge,Update}, but process a sequence-of-2-
sequences argument instead of a mapping object.  For now, I left these
functions file static, so no corresponding doc changes.  It's tempting
to change dict.update() to allow a sequence-of-2-seqs argument too.

Also changed the name of dictionary's keyword argument from "mapping"
to "x".  Got a better name?  "mapping_or_sequence_of_pairs" isn't
attractive, although more so than "mosop" <wink>.

abstract.h, abstract.tex:  Added new PySequence_Fast_GET_SIZE function,
much faster than going thru the all-purpose PySequence_Size.

libfuncs.tex:
- Document dictionary().
- Fiddle tuple() and list() to admit that their argument is optional.
- The long-winded repetitions of "a sequence, a container that supports
  iteration, or an iterator object" is getting to be a PITA.  Many
  months ago I suggested factoring this out into "iterable object",
  where the definition of that could include being explicit about
  generators too (as is, I'm not sure a reader outside of PythonLabs
  could guess that "an iterator object" includes a generator call).
- Please check my curly braces -- I'm going blind <0.9 wink>.

abstract.c, PySequence_Tuple():  When PyObject_GetIter() fails, leave
its error msg alone now (the msg it produces has improved since
PySequence_Tuple was generalized to accept iterable objects, and
PySequence_Tuple was also stomping on the msg in cases it shouldn't
have even before PyObject_GetIter grew a better msg).

Doc/api/abstract.tex

@@ -125,7 +125,7 @@ for which they do not apply, they will raise a Python exception.
   the Unicode string representation on success, \NULL{} on failure.
   This is the equivalent of the Python expression
   \samp{unistr(\var{o})}.  Called by the
-  \function{unistr()}\bifuncindex{unistr} built-in function. 
+  \function{unistr()}\bifuncindex{unistr} built-in function.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{int}{PyObject_IsInstance}{PyObject *inst, PyObject *cls}
@@ -715,10 +715,17 @@ determination.
 
 \begin{cfuncdesc}{PyObject*}{PySequence_Fast_GET_ITEM}{PyObject *o, int i}
   Return the \var{i}th element of \var{o}, assuming that \var{o} was
-  returned by \cfunction{PySequence_Fast()}, and that \var{i} is
-  within bounds.  The caller is expected to get the length of the
-  sequence by calling \cfunction{PySequence_Size()} on \var{o}, since
-  lists and tuples are guaranteed to always return their true length.
+  returned by \cfunction{PySequence_Fast()}, \var{o} is not \NULL{},
+  and that \var{i} is within bounds.
+\end{cfuncdesc}
+
+\begin{cfuncdesc}{int}{PySequence_Fast_GET_SIZE}{PyObject *o}
+  Returns the length of \var{o}, assuming that \var{o} was
+  returned by \cfunction{PySequence_Fast()} and that \var{o} is
+  not \NULL{}.  The size can also be gotten by calling
+  \cfunction{PySequence_Size()} on \var{o}, but
+  \cfunction{PySequence_Fast_GET_SIZE()} is faster because it can
+  assume \var{o} is a list or tuple.
 \end{cfuncdesc}