Add mutations_edge array access

[benjeffery]

Fixes #3189

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.61%. Comparing base (da094c4) to head (27552de).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
python/_tskitmodule.c	86.66%	1 Missing and 1 partial ⚠️
python/tskit/trees.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3226      +/-   ##
==========================================
- Coverage   89.61%   89.61%   -0.01%     
==========================================
  Files          28       28              
  Lines       31895    31916      +21     
  Branches     5872     5876       +4     
==========================================
+ Hits        28583    28601      +18     
- Misses       1884     1885       +1     
- Partials     1428     1430       +2     

Flag	Coverage Δ
c-tests	86.59% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	88.18% <86.66%> (-0.01%)	⬇️
python-tests	98.85% <83.33%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
python/tskit/trees.py	98.85% <83.33%> (-0.04%)	⬇️
python/_tskitmodule.c	88.18% <86.66%> (-0.01%)	⬇️

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