RFR: 8314169: Combine related RoundingMode logic in j.text.DigitList [v3]

[Justin Lu]

> Please review this PR which is a broad clean up of the DigitList class (used by Format classes in j.text).
> 
> This PR is intended to be a portion of a bigger change (split up to make reviewing easier). 
> 
> The main change simplifying related Rounding Mode logic in `shouldRoundUp()` - (_CEILING/FLOOR_, _HALF_UP/DOWN/EVEN_)
> 
> Other changes include
> - Certain for loops can be replaced with cleaner syntax (E.g. for(;;), empty for loops)
> - Introduce `roundInt(int)` - For use by Integer representations of DigitList
> - Introduce `nonZeroAfterIndex(int)` - To reduce code duplication

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Reflect review comment: uncombine CEILING/FLOOR (8/15/23)

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15252/files
  - new: https://git.openjdk.org/jdk/pull/15252/files/c9782b77..dc9dcf33

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15252&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15252&range=01-02

  Stats: 9 lines in 1 file changed: 1 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15252.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15252/head:pull/15252

PR: https://git.openjdk.org/jdk/pull/15252