RFR: 8305811: (bf) Improve heap buffer performance of CharBuffer::append(CharSequence) [v3]

[Brian Burkhalter]

> Use the `getChars` method of `String`, `StringBuffer`, and `StringBuilder` to load the chars directly into the array of the heap buffer.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8305811: Cleanup to address reviewer comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13415/files
  - new: https://git.openjdk.org/jdk/pull/13415/files/67025b90..d980ce22

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13415&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13415&range=01-02

  Stats: 11 lines in 2 files changed: 3 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13415/head:pull/13415

PR: https://git.openjdk.org/jdk/pull/13415