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Here is another patch.  This one focuses on simplicity, both
implementation-wise and from the user's point of view:

1) It only makes repeated  variable += expr  faster. It
doesn't touch the '+'.
2) It doesn't mess with the internals of strings and dicts
any more.  It is just one well-documented function now.

The goal of this new patch is to be reviewable and
maintainable, to get it in the core to stop people from
being bitten by the performance of += (I just saw yet
another example yesterday).