RFR: 8306374: (bf) Improve performance of DirectCharBuffer::append(CharSequence[,int,int])

[Brian Burkhalter]

Use `getChars()` to copy characters to an intermediate local `char[]` array and then `putArray()` to copy the chars into the buffer itself.

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Commit messages:
 - 8306374: (bf) Improve performance of DirectCharBuffer::append(CharSequence[,int,int])

Changes: https://git.openjdk.org/jdk/pull/13522/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13522&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306374
  Stats: 99 lines in 3 files changed: 97 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13522.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13522/head:pull/13522

PR: https://git.openjdk.org/jdk/pull/13522