RFR: 8349525: RBTree: provide leftmost, rightmost, and a simple way to print trees

[Thomas Stuefe]

For things I currently work on (compilation memory statistic), I need this functionality.

Changes:

- added leftmost() and rightmost() (pretty self-explanatory)
- added print_on(outputStream*) (likewise)
- const correctness
- other minor cleanups
- gtests for all added functions

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Commit messages:
 - fix constness, lp64 issues
 - RBTree: leftmost, rightmost, print_on

Changes: https://git.openjdk.org/jdk/pull/23486/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23486&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349525
  Stats: 262 lines in 3 files changed: 187 ins; 9 del; 66 mod
  Patch: https://git.openjdk.org/jdk/pull/23486.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23486/head:pull/23486

PR: https://git.openjdk.org/jdk/pull/23486