[issue2799] Remove PyUnicode_AsString(), rework PyUnicode_AsStringAndSize(), add PyUnicode_AsChar()

[Haoyu Bai]

Changes by Haoyu Bai <divinekid at gmail.com>:


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nosy: +bhy

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Tracker <report at bugs.python.org>
<http://bugs.python.org/issue2799>
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