RFR: 8361209: (bf) Use CharSequence::getChars for StringCharBuffer bulk get methods

[Brian Burkhalter]

For `CharBuffer`s created from a `CharSequence`, use `CharSequence.getChars` to perform bulk gets instead of successively copying single characters.

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Commit messages:
 - 8361209: (bf) Use CharSequence::getChars for StringCharBuffer bulk get methods

Changes: https://git.openjdk.org/jdk/pull/26251/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26251&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361209
  Stats: 94 lines in 2 files changed: 92 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26251.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26251/head:pull/26251

PR: https://git.openjdk.org/jdk/pull/26251