[issue18813] Speed up slice object processing

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> There could be two implementations of "slice", one that
> uses Python object indices (as currently) and one that has Py_ssize_t
> indices  (and properties for the start/stop/step attributes).

Agree, this idea LGTM. Single boolean flag should be enough to switch between 
implementations. This shouldn't break well written code. The PySliceObject 
structure is not a part of stable API.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18813>
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