RFR: 8315585: Optimization for decimal to string

[Shaojin Wen]

Continue to complete PR #16006 to improve performance and reduce duplicate code

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Commit messages:
 - optimize BigDecimal toString/toPlainString
 - add benchmark

Changes: https://git.openjdk.org/jdk/pull/22009/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315585
  Stats: 391 lines in 3 files changed: 188 ins; 121 del; 82 mod
  Patch: https://git.openjdk.org/jdk/pull/22009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22009/head:pull/22009

PR: https://git.openjdk.org/jdk/pull/22009