fix: clean up #[allow(clippy::from_over_into)]

[metatoaster]

Using the From trait is more idiomatic, and not to mention it allows the conversion to be invoked directly via the target type's from:

let arena = &mut Arena::new();
let a = arena.new_node(1);
dbg!(usize::from(a));
dbg!(NonZeroUsize::from(a));
dbg!(usize::from(a) as u64)

Rather than going through a much more verbose syntax as required currently, especially when a u64 is desired.

dbg!(<NodeId as Into<usize>>::into(a));
dbg!(<NodeId as Into<usize>>::into(a) as u64);

[saschagrunert]

@metatoaster thank you for the PR, do you mind a rebase to fixup the CI?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.8%. Comparing base (c912895) to head (57ecdb5).
Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #131      +/-   ##
========================================
+ Coverage   54.5%   87.8%   +33.2%     
========================================
  Files          9       9              
  Lines        559    1186     +627     
  Branches     188       0     -188     
========================================
+ Hits         305    1042     +737     
- Misses       135     144       +9     
+ Partials     119       0     -119     

[metatoaster]

Threw in conversion tests at the last minute for coverage completeness sake. Thanks for looking into these changes.