RFR: 8339317: Optimize ClassFile writeBuffer [v6]

[Shaojin Wen]

> A small optimization, optimize the BufferWriter implementation and use of ClassFile, provide faster patchInt and skip

Shaojin Wen has updated the pull request incrementally with two additional commits since the last revision:

 - Eliminate redundant variables
 - buf skip

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20780/files
  - new: https://git.openjdk.org/jdk/pull/20780/files/e3dc3014..21f0a229

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20780&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20780&range=04-05

  Stats: 25 lines in 4 files changed: 12 ins; 3 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/20780.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20780/head:pull/20780

PR: https://git.openjdk.org/jdk/pull/20780