get node id from index with arena

[coderedart]

closes #101

Its a pretty simple PR. Mainly useful when you don't care if its the "same value", but just that you can get the node id back from the index.

[codecov]

Codecov Report

Merging #102 (90f582a) into main (a0e699f) will increase coverage by 1.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #102     +/-   ##
=======================================
+ Coverage   59.7%   60.7%   +1.0%     
=======================================
  Files          8       8             
  Lines        477     474      -3     
  Branches     171     170      -1     
=======================================
+ Hits         285     288      +3     
+ Misses        50      42      -8     
- Partials     142     144      +2     

[saschagrunert]

Thanks!