Iterators phase 1.  This comprises:

new slot tp_iter in type object, plus new flag Py_TPFLAGS_HAVE_ITER
new C API PyObject_GetIter(), calls tp_iter
new builtin iter(), with two forms: iter(obj), and iter(function, sentinel)
new internal object types iterobject and calliterobject
new exception StopIteration
new opcodes for "for" loops, GET_ITER and FOR_ITER (also supported by dis.py)
new magic number for .pyc files
new special method for instances: __iter__() returns an iterator
iteration over dictionaries: "for x in dict" iterates over the keys
iteration over files: "for x in file" iterates over lines

TODO:

documentation
test suite
decide whether to use a different way to spell iter(function, sentinal)
decide whether "for key in dict" is a good idea
use iterators in map/filter/reduce, min/max, and elsewhere (in/not in?)
speed tuning (make next() a slot tp_next???)
diff --git a/Lib/dis.py b/Lib/dis.py
index dc30257..fb97369 100644
--- a/Lib/dis.py
+++ b/Lib/dis.py
@@ -205,6 +205,7 @@
 def_op('BINARY_XOR', 65)
 def_op('BINARY_OR', 66)
 def_op('INPLACE_POWER', 67)
+def_op('GET_ITER', 68)
 
 def_op('PRINT_EXPR', 70)
 def_op('PRINT_ITEM', 71)
@@ -232,6 +233,7 @@
 name_op('STORE_NAME', 90)       # Index in name list
 name_op('DELETE_NAME', 91)      # ""
 def_op('UNPACK_SEQUENCE', 92)   # Number of tuple items
+def_op('FOR_ITER', 93)
 
 name_op('STORE_ATTR', 95)       # Index in name list
 name_op('DELETE_ATTR', 96)      # ""