RFR: 8343829: Unify decimal and hexadecimal parsing in FloatingDecimal

[Raffaello Giulietti]

See the JBS bug for some details.

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Commit messages:
 - Refactoring some code and comments.
 - 8343829: Unify decimal and hexadecimal parsing in FloatingDecimal

Changes: https://git.openjdk.org/jdk/pull/22737/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343829
  Stats: 1021 lines in 1 file changed: 334 ins; 456 del; 231 mod
  Patch: https://git.openjdk.org/jdk/pull/22737.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22737/head:pull/22737

PR: https://git.openjdk.org/jdk/pull/22737