[issue31108] add __contains__ for list_iterator (and others) for better performance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Terry, this proposition doesn't change the behavior. It moves the iterator forward during searching. The only effect is inlining __next__ in a loop and getting rid of the overhead of few indirections and calls.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31108>
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