[Patch] Optimize _BFSExecutor in regex

[Tim Shen]

Here's a simple piece of code
https://gist.github.com/innocentim/6849759 the reveals _BFSExecutor's
inefficiency. Some optimizations are here to reduce the unecessary
time complexity from O(n^2) to O(n).

I'll do a bootstrap and full testing before committing.

Thanks!


-- 
Tim Shen
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