RFR: 8342650: Move getChars to DecimalDigits [v4]

[Shaojin Wen]

> Move getChars methods of StringLatin1 and StringUTF16 to DecimalDigits to reduce duplication
> 
> 1. HexDigits and OctalDigits also include getCharsLatin1 and getCharsUTF16
> 2. Putting these two methods into DecimalDigits can avoid the need to expose them in JavaLangAccess
> 3. Eliminate duplicate code in BigDecimal

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  add benchmark

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21593/files
  - new: https://git.openjdk.org/jdk/pull/21593/files/e97138db..cf947f61

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21593&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21593&range=02-03

  Stats: 50 lines in 1 file changed: 50 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21593.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21593/head:pull/21593

PR: https://git.openjdk.org/jdk/pull/21593