RFR: 8316150: Refactor get chars and string size [v9]

[温绍锦]

> 1. Reduce duplicate stringSize code
> 2. Move java.lang.StringLatin1.getChars to jdk.internal.util.DecimalDigits::getCharLatin1，not only java.lang, other packages also need to use this method

温绍锦 has updated the pull request incrementally with one additional commit since the last revision:

  refactor HexDigits & OctalDigits & FormatItem, FormatItem#prepend provides two implementations: prependLatin1 and prependUTF16 [v2]

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15699/files
  - new: https://git.openjdk.org/jdk/pull/15699/files/f1cb9fd7..074c48ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15699&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15699&range=07-08

  Stats: 97 lines in 5 files changed: 73 ins; 14 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15699/head:pull/15699

PR: https://git.openjdk.org/jdk/pull/15699