RFR: 8303882: Refactor some iterators in jdk.compiler [v2]

[Pavel Rappo]

> Please review this refactoring to iterators in jdk.compiler. The refactoring delegates more to collections framework, while retaining the performance characteristics of bespoke iterators. The refactoring also adds inline comments, `@Override` annotations, and fixes some trivial bugs (as can be seen in the added test, which is slightly augmented from that suggested by Jan Lahoda here https://github.com/openjdk/jdk/pull/12904#discussion_r1129777660).
> 
> I'll add a comment here once I have benchmarked the change formally.

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  Update test/langtools/tools/javac/util/IteratorsTest.java
  
  Co-authored-by: Andrey Turbanov <turbanoff at gmail.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12949/files
  - new: https://git.openjdk.org/jdk/pull/12949/files/6c3eeb13..8df4ed07

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12949&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12949&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12949.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12949/head:pull/12949

PR: https://git.openjdk.org/jdk/pull/12949