RFR: 8315585: Optimization for decimal to string

[Shaojin Wen]

Continue to complete PR #16006 and PR #21593 to improve BigDecimal::toString and BigDecimal::toPlainString performance and reduce duplicate code

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Commit messages:
 - simplify and comments
 - simplify
 - simplify
 - code style
 - revert change
 - bug fix
 - bug fix
 - refactor
 - Revert "simplify scale2"
 - simplify scale2
 - ... and 15 more: https://git.openjdk.org/jdk/compare/f446cefe...cb88e0ef

Changes: https://git.openjdk.org/jdk/pull/23310/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23310&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315585
  Stats: 448 lines in 3 files changed: 237 ins; 118 del; 93 mod
  Patch: https://git.openjdk.org/jdk/pull/23310.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23310/head:pull/23310

PR: https://git.openjdk.org/jdk/pull/23310