Faster `SwapMatrixRows` and `SwapMatrixColumns` for 8bit matrices

[fingolfin]

Before:

gap> mat:=IdentityMat(10,GF(5));; ConvertToMatrixRep(mat);; mat;
< mutable compressed matrix 10x10 over GF(5) >
gap> for i in [1..1000000] do SwapMatrixColumns(mat,1,2); od; time;
1308
gap> for i in [1..1000000] do SwapMatrixRows(mat,1,2); od; time;
125
gap> for i in [1..1000000] do tmp:=mat[1];mat[1]:=mat[2];mat[2]:=tmp; od; time;
92

After:

gap> mat:=IdentityMat(10,GF(5));; ConvertToMatrixRep(mat);; mat;
< mutable compressed matrix 10x10 over GF(5) >
gap> for i in [1..1000000] do SwapMatrixColumns(mat,1,2); od; time;
198
gap> for i in [1..1000000] do SwapMatrixRows(mat,1,2); od; time;
28
gap> for i in [1..1000000] do tmp:=mat[1];mat[1]:=mat[2];mat[2]:=tmp; od; time;
92

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