RFR: 8305811: (bf) Improve heap buffer performance of CharBuffer::append(CharSequence) [v2]

[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: Refactor append methods

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13415/files
  - new: https://git.openjdk.org/jdk/pull/13415/files/0cfc179b..67025b90

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13415&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13415&range=00-01

  Stats: 55 lines in 2 files changed: 31 ins; 21 del; 3 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