RFR: JDK-8302323 Add repeat methods to StringBuilder/StringBuffer

[Jim Laskey]

Add the ability to repeatedly append char and CharSequence data to StringBuilder/StringBuffer.

-------------

Commit messages:
 - Clean up fro CSR
 - Add synchronized to StringBuffer repeats
 - AbstractStringBuilder repeat

Changes: https://git.openjdk.org/jdk/pull/12728/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302323
  Stats: 340 lines in 5 files changed: 336 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12728.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12728/head:pull/12728

PR: https://git.openjdk.org/jdk/pull/12728