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

[Brian Burkhalter]

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

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Commit messages:
 - 8305811: (bf) Improve heap buffer performance of CharBuffer::append(CharSequence)
 - Improve performance of CharBuffer::append(CharSequence)

Changes: https://git.openjdk.org/jdk/pull/13415/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305811
  Stats: 41 lines in 1 file changed: 40 ins; 0 del; 1 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