[issue2799] Remove _PyUnicode_AsString(), rework _PyUnicode_AsStringAndSize(), add PyUnicode_AsChar()

[Alexander Belopolsky]

Changes by Alexander Belopolsky <belopolsky at users.sourceforge.net>:


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components: +Interpreter Core
stage:  -> needs patch
versions: +Python 3.3 -Python 3.0, Python 3.1

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<http://bugs.python.org/issue2799>
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