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).

Include/abstract.h

@@ -951,26 +951,30 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx*/
 
 
      DL_IMPORT(PyObject *) PySequence_List(PyObject *o);
-
        /*
 	 Returns the sequence, o, as a list on success, and NULL on failure.
 	 This is equivalent to the Python expression: list(o)
        */
 
      DL_IMPORT(PyObject *) PySequence_Fast(PyObject *o, const char* m);
-
        /*
          Returns the sequence, o, as a tuple, unless it's already a
          tuple or list.  Use PySequence_Fast_GET_ITEM to access the
-         members of this list.
+         members of this list, and PySequence_Fast_GET_SIZE to get its length.
 
          Returns NULL on failure.  If the object does not support iteration,
          raises a TypeError exception with m as the message text.
        */
 
+#define PySequence_Fast_GET_SIZE(o) \
+	(PyList_Check(o) ? PyList_GET_SIZE(o) : PyTuple_GET_SIZE(o))
+       /*
+	 Return the size of o, assuming that o was returned by
+         PySequence_Fast and is not NULL.
+       */
+
 #define PySequence_Fast_GET_ITEM(o, i)\
      (PyList_Check(o) ? PyList_GET_ITEM(o, i) : PyTuple_GET_ITEM(o, i))
-
        /*
 	 Return the ith element of o, assuming that o was returned by
          PySequence_Fast, and that i is within bounds.