Slicing support in Python

This patch (attached, works with r81) adds slicing support for repeated fields in Python.  It implements retrieval, setting, and deletion for both scalar and composite repeated fields, and adds several unit tests for each method.

There was already a remove() method for repeated scalars, but not for composites.  I added one without any difficulty.  Did I miss something?  In fact, this whole patch was pretty easy.  There must be some reason it hasn't been done before.

The __eq__() method for repeated composites disallows comparing to any other sequence type, including ordinary lists.  Why can't we do this?

Adding more methods to the _RepeatedScalarFieldContainer and _RepeatedCompositeFieldContainer classes makes it even more painfully obvious that they need to be merged.  I'm sure we can make it polymorphic, and that'll probably be my next patch.

[Alek Storm, 2008-12-02 20:34:57]

Huh, the side-by-side diffs feature doesn't work. The repo doesn't have an
'svn-history' directory.

[kenton, 2008-12-02 21:05:50]

Strange, many people have used this successfully before...

[Alek Storm, 2008-12-03 00:44:56]

Okay, *now* it's working. Go ahead.

[kenton, 2008-12-03 01:16:12]

On 2008/12/03 00:44:56, Alek Storm wrote:
> Okay, *now* it's working. Go ahead.

Cool.  Petar, can you take a look at this?

[Alek Storm, 2008-12-18 16:32:59]

Removed __setslice__ method for composites

[Alek Storm, 2008-12-20 03:59:57]

Okay, removed all assignment methods and remove() for composites

[pesho.petrov, 2008-12-20 04:19:38]

Looks Good!

http://codereview.appspot.com/8882/diff/605/403
File python/google/protobuf/internal/reflection_test.py (right):

http://codereview.appspot.com/8882/diff/605/403#newcode59
Line 59: class ReflectionTest(unittest.TestCase):
Thanks! I never noticed this typo.