RFR: 8349525: RBTree: provide leftmost, rightmost, and a simple way to print trees [v3]

[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
> 
> Tests: GHA (all clean), manual tests on Linux x64

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  feedback caspar

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23486/files
  - new: https://git.openjdk.org/jdk/pull/23486/files/c96cfa35..4f9eddb4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23486&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23486&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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