[MERGE] faster iter_changes

[Robert Collins]

This adds a pyrex accelerator object supporting iter_changes. This
combined with the improved walkdirs code drops status on a mozilla tree
from 3.84 seconds to 2.37 seconds (hot cache, plugins enabled).

Broadly, I've added a coupled of C extension points, and ensured they
have tests running both C and python versions. The extension points
names can probably be repainted, but as everything is private anyhow,
I'm not terribly stressed.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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