[MERGE] _KnitIndex._load_data in pyrex

[Martin Pool]

I haven't read the full patch yet, but it does seem that we should
settle a standard way to load pyrex.

Since loading Python code does have some cost it would be nice to
avoid loading the Python implementation if we're going to use the
pyrex one.  I also agree with John that it is desirable that it be
possible to load both simultaneously for testing.

I would suggest maybe we write a paragraph or two for HACKING saying
what the plan is.

-- 
Martin