Implement DoubleEndedIterator for some iterator structs

[alexmozaidze]

Note

This PR is a semi-draft; I want to hear some feedback before I start refining the code.

Changes:

1. Implemented DoubleEndedIterator for some of the iterator structs
2. Reduced repetitiveness in traverse.rs using a new macro that replaces impl_node_iterator! completely
3. Removed FusedIterator for everything using the new macro, just to be safe (may be re-implemented if the current changes adhere to FusedIterator's contract)
4. Deprecated ReverseChildren since Children::rev() is more idiomatic.

There is an edge-case for when I need to get first/last root node from the arena, in which case I resorted to iter().next() and iter().last(), which I don't think is efficient. If you have any suggestions, I'd be happy to hear them.

[alexmozaidze]

I may be able to ensure FusedIterator's contract by bringing back old behaviour for iterators that don't implement DoubleEndedIterator. I'll make some adjustments to the macro.

[alexmozaidze]

Yep, I think it's all done now.

Macro is convenient, code duplication is close to none, and there are no feature regressions for one way iterators (FusedIterator implementation).

Unless there are optimizations I've missed that can make some iterators faster, there is nothing else to add.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.3%. Comparing base (cd3d680) to head (9d054f7).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #107     +/-   ##
=======================================
- Coverage   66.0%   65.3%   -0.8%     
=======================================
  Files          8       8             
  Lines        474     467      -7     
  Branches     145     142      -3     
=======================================
- Hits         313     305      -8     
- Misses        42      44      +2     
+ Partials     119     118      -1     

[saschagrunert]

May I ask you to fix the CI issues? :)

[alexmozaidze]

The clippy one should be simple to fix (#[allow(deprecated)]), but I am not sure about how to fix the code coverage. How do I fix it? Do I add tests?

[saschagrunert]

The clippy one should be simple to fix (#[allow(deprecated)]), but I am not sure about how to fix the code coverage. How do I fix it? Do I add tests?

The coverage is no concern, I'm mostly speaking about the clippy ones.

[saschagrunert]

Thank you, code LGTM!

[saschagrunert]

A bunch of tests would be nice, but I don't see it as a must here.

[alexmozaidze]

I will add some tests in a future PR, along with other improvements.