RFR: 8229845: Decrease memory consumption of BigInteger.toString()

[Brian Burkhalter]

Please review this proposed fix. The review was initially in this thread

http://mail.openjdk.java.net/pipermail/core-libs-dev/2019-August/061914.html

under the old Hg SCM. The changes proposed here are derived from the final (.05) patch in the previous review which was posted in

http://mail.openjdk.java.net/pipermail/core-libs-dev/2019-August/062012.html.

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Commit messages:
 - 8229845: Decrease memory consumption of BigInteger.toString()

Changes: https://git.openjdk.java.net/jdk/pull/819/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=819&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8229845
  Stats: 115 lines in 2 files changed: 57 ins; 21 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/819.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/819/head:pull/819

PR: https://git.openjdk.java.net/jdk/pull/819