improve orbit_representatives_and_stabilizers

[ThomasBreuer]

On the GAP side, replace the action on subspces by an equivalent action on matrices.

For not too large examples, this yields a speedup.
Big examples suffer from the approach to write down the whole set.
(In the end, we want only orbit representatives.)

[ThomasBreuer]

Failure in test (1.10, short, ubuntu-latest):

GC: pause 42.41ms. collected 32.954742MB. incr 
corrupted size vs. prev_size while consolidating

[2001] signal (6.-6): Aborted
in expression starting at /home/runner/work/Oscar.jl/Oscar.jl/experimental/OrthogonalDiscriminants/test/theoretical.jl:53
Error: The operation was canceled.

Restarted this test.

[fingolfin]

To really get this fast we need a fresh dedicated implementation of subspace stabilizers / representatives, by exploiting the module structure (using the Meataxe). But of course that takes more time so a quick minor improvement is still useful

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.50%. Comparing base (5a737af) to head (000906c).
Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4337   +/-   ##
=======================================
  Coverage   84.50%   84.50%           
=======================================
  Files         645      645           
  Lines       85621    85624    +3     
=======================================
+ Hits        72354    72357    +3     
  Misses      13267    13267           

Files with missing lines	Coverage Δ
src/Groups/gsets.jl	92.35% <100.00%> (+0.07%)	⬆️

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