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

[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 combines 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 overloaded `round(int)` - For use by Integer representations of DigitList
- Introduce `non0AfterIndex(int)` - To reduce code duplication

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

Commit messages:
 - Init

Changes: https://git.openjdk.org/jdk/pull/15252/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314169
  Stats: 166 lines in 1 file changed: 59 ins; 68 del; 39 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