Remove many calls to *cat(::Vector{...})

[lgoettgens]

To prepare for Nemocas/AbstractAlgebra.jl#1467

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (d8aa190) 74.55% compared to head (f7611e1) 74.56%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1253   +/-   ##
=======================================
  Coverage   74.55%   74.56%           
=======================================
  Files         346      346           
  Lines      110993   110993           
=======================================
+ Hits        82756    82765    +9     
+ Misses      28237    28228    -9     

Files	Coverage Δ
src/AlgAssAbsOrd/LocallyFreeClassGroup.jl	92.30% <100.00%> (ø)
src/GenOrd/Ideal.jl	78.32% <100.00%> (ø)
src/GrpAb/Map.jl	79.78% <100.00%> (ø)
src/Map/GrpAb.jl	80.23% <100.00%> (ø)
src/Misc/Poly.jl	62.11% <100.00%> (ø)
src/NumFieldOrd/NfOrd/norm_eqn.jl	76.00% <100.00%> (ø)
src/GenOrd/GenOrd.jl	78.30% <50.00%> (ø)
src/GenOrd/Types.jl	88.78% <0.00%> (ø)
src/LocalField/neq.jl	68.37% <50.00%> (-0.86%)	⬇️
src/Misc/PseudoPolynomial.jl	9.72% <0.00%> (ø)
... and 4 more

... and 26 files with indirect coverage changes

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