RFR: 8303882: Refactor some iterators in jdk.compiler [v3]

[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 four additional commits since the last revision:

 - Merge remote-tracking branch 'myjdk/8303882' into 8303882
 - Squeeze a few extra drops of perf
 - Add a recursive test
 - Try squeeze more performance

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12949/files
  - new: https://git.openjdk.org/jdk/pull/12949/files/8df4ed07..04596f39

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12949&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12949&range=01-02

  Stats: 28 lines in 2 files changed: 15 ins; 6 del; 7 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