Start of a mergeable ListItemIterator. Breaks tons of stuff, but annotation
passes

Files:
modified: /pypy/branch/tuple-nonresizable-395/pypy/annotation/model.py (try)
modified: /pypy/branch/tuple-nonresizable-395/pypy/annotation/binaryop.py (try)
modified: /pypy/branch/tuple-nonresizable-395/pypy/annotation/listdef.py (try)
modified: /pypy/branch/tuple-nonresizable-395/pypy/annotation/unaryop.py (try)
modified: /pypy/branch/tuple-nonresizable-395/pypy/annotation/test/test_annrpython.py (try)