RFR: 8366478: BigDecimal roots [v21]

[fabioromano1]

> An implementation of `BigDecimal.nthRoot(int, MathContext)`.

fabioromano1 has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 25 commits:

 - Merge branch 'openjdk:master' into 8366478
 - Code optimization
 - Add comment
 - Speed up root inversion by computing result's scale before inversion
 - Ensure correct precision for rounding and simplify code
 - Correct rounding procedure when n < 0
 - Small corrections
 - Correct comment
 - Correct procedure for rounding when n < 0
 - Correct inversion of radicand
 - ... and 15 more: https://git.openjdk.org/jdk/compare/7bb490c4...d64ac405

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Changes: https://git.openjdk.org/jdk/pull/27148/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27148&range=20
  Stats: 338 lines in 2 files changed: 139 ins; 127 del; 72 mod
  Patch: https://git.openjdk.org/jdk/pull/27148.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27148/head:pull/27148

PR: https://git.openjdk.org/jdk/pull/27148