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This version is tidied up enough that I think it could be reviewed.
Changes are:
 * Defer `buf' allocation until __init__, rather than __new__ as was previously done. Now upon completion, BytesIO.__new__ returns a valid, closed BytesIO, whereas previously a valid, empty, open BytesIO was returned. Is this interface change permissible?
 * Move __init__ guts into a "reinit()", for sharing with __setstate__, which also previously caused an unnecessary copy. Additionally gather up various methods for deallocating buffers into a single "reset()" function, called by reinit(), _dealloc(), and _close()
 * Per Stefan's suggested approach, reinit() now explicitly checks for a read-only buffer, falling back to silently performing a copy if the returned buffer is read-write. That seems vastly preferable to throwing an exception, which would probably be another interface change.
 * Call `unshare()` any place the buffer is about to be modified. If the buffer needs to be made private, it also accepts a size hint indicating how much less/more space the subsequent operation needs, to avoid a redundant reallocation after the unsharing.
Outstanding issues:
 * I don't understand why buf_size is a size_t, and I'm certain the casting in unshare() is incorrect somehow. Is it simply to avoid signed overflow?