Widen declaration for many matrix related functions

[fingolfin]

The main motivation of course is to be able to use more MatrixObj stuff.

Also remove some now redundant methods.

Before:

gap> m:=RandomMat(256, 256, GF(1000000007));;
gap> mat:=Matrix(IsZmodnZMatrixRep, GF(1000000007), m);;
gap> RankMat(mat)=256; time;
true
170
gap> DeterminantMat(mat); time;
ZmodpZObj( 873487643, 1000000007 )
23982

After:

gap> m:=RandomMat(256, 256, GF(1000000007));;
gap> mat:=Matrix(IsZmodnZMatrixRep, GF(1000000007), m);;
gap> RankMat(mat)=256; time;
true
161
gap> DeterminantMat(mat); time;
ZmodpZObj( 873487643, 1000000007 )
1949

As discussed on #6088

[wilfwilson]

Seems fine to me.

The diffs in the manual examples are harmless, as far as I can tell (the relevant groups are equal, they're just given by different generating sets).

[wilfwilson]

There's still one pesky harmless failing manual example 🙁

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.88%. Comparing base (74a43d6) to head (c6d1276).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6095      +/-   ##
==========================================
+ Coverage   78.86%   78.88%   +0.01%     
==========================================
  Files         685      685              
  Lines      293550   293542       -8     
  Branches     8672     8672              
==========================================
+ Hits       231515   231556      +41     
+ Misses      60225    60177      -48     
+ Partials     1810     1809       -1     

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