RFR: 8311175: Move BufWriter::asByteBuffer to BufWriterImpl

[Adam Sotona]

On Fri, 30 Jun 2023 14:43:36 GMT, Chen Liang <liach at openjdk.org> wrote:

> As discussed on the mailing list https://mail.openjdk.org/pipermail/classfile-api-dev/2023-June/000381.html, BufWriter::asByteBuffer has a behavior not suitable for API and is only used by internal StackMapGenerator/StackCounter, so it will be converted to an internal API.
> 
> Somehow the ByteBuffer needs to be sliced, or StackMapGenerator encounters IOOBE. Not sure what the exact cause was.

@liach Do you have any plans with this PR?

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PR Comment: https://git.openjdk.org/jdk/pull/14736#issuecomment-2097490371