RFR: 8311175: Move BufWriter::asByteBuffer to BufWriterImpl [v3]

[Chen Liang]

> 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.

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  Redundant import, copyright year

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14736/files
  - new: https://git.openjdk.org/jdk/pull/14736/files/d82789c3..8830ca25

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14736&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14736&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14736.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14736/head:pull/14736

PR: https://git.openjdk.org/jdk/pull/14736