Add ShallowCopy for PrimitiveGroupsIterator

[fingolfin]

List on an iterator iterates over a ShallowCopy of it, and the iterator returned by PrimitiveGroupsIterator had no such method, so List failed with "no method found".

Fixes #102

Assisted-by: Claude Code (Opus 5.5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.62%. Comparing base (f62f6be) to head (91ea41f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   99.61%   99.62%           
=======================================
  Files          52       52           
  Lines       44793    44805   +12     
=======================================
+ Hits        44622    44636   +14     
+ Misses        171      169    -2     

Files with missing lines	Coverage Δ
lib/primitiv.gi	90.93% <100.00%> (+0.41%)	⬆️

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