Fix *cat inconsistencies

[nrontsis]

This PR addresses #113 and #168 by making cats support arbitrarily many arguments, and making *cat(args...) equivalent to reduce(*cat, args...).

Tests were extended accordingly.

[nrontsis]

@jonniedie I fixed the tests here as well, after incorporating changes from #170. So perhaps first review #170.

[codecov-commenter]

Codecov Report

Merging #169 (5f92105) into master (cbb24ef) will increase coverage by 2.30%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   73.49%   75.79%   +2.30%     
==========================================
  Files          20       20              
  Lines         679      599      -80     
==========================================
- Hits          499      454      -45     
+ Misses        180      145      -35     

Impacted Files	Coverage Δ
src/broadcasting.jl	100.00% <ø> (ø)
src/componentarray.jl	81.30% <ø> (-0.59%)	:arrow_down:
src/show.jl	3.84% <ø> (ø)
src/array_interface.jl	86.07% <100.00%> (+2.91%)	:arrow_up:
src/axis.jl	84.41% <100.00%> (-1.30%)	:arrow_down:
src/compat/gpuarrays.jl	72.91% <100.00%> (+20.03%)	:arrow_up:
src/linear_algebra.jl	95.83% <100.00%> (-0.33%)	:arrow_down:
src/similar_convert_copy.jl	75.00% <0.00%> (-10.00%)	:arrow_down:
src/utils.jl	93.93% <0.00%> (-3.04%)	:arrow_down:
... and 2 more

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