[issue17742] Add _PyBytesWriter API

[STINNER Victor]

STINNER Victor added the comment:

New version of the patch:
 - address most (all?) Serhiy's remarks
 - _PyBytesWriter_PrepareInternal() always use min_size, not only when overallocate is 1
 - add more comments

Performances are almost the same than without the patch. It looks like they are a little bit better.

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Added file: http://bugs.python.org/file29931/bytes_writer-2.patch

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