Fix linking bug which causes a stack overflow on a thread when actors are bi-directionally linked

[slawlor]

Resolves #167

[codecov]

Codecov Report

Patch coverage: 91.89% and project coverage change: +0.04% 🎉

Comparison is base (0761b8c) 78.91% compared to head (8a8b02c) 78.95%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   78.91%   78.95%   +0.04%     
==========================================
  Files          49       49              
  Lines        8945     8977      +32     
==========================================
+ Hits         7059     7088      +29     
- Misses       1886     1889       +3     

Files Changed	Coverage Δ
ractor/src/actor/tests/supervisor.rs	91.71% <88.88%> (-0.09%)	⬇️
ractor/src/actor/supervision.rs	97.77% <100.00%> (+0.27%)	⬆️

... and 1 file with indirect coverage changes

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