Fix hcat and vcat for variables arguments

[abelsiqueira]

Closes #97

[coveralls]

Coverage increased (+0.2%) to 92.465% when pulling 4052165 on abelsiqueira:fix-cat into 3462b64 on JuliaSmoothOptimizers:master.

[codecov]

Codecov Report

Merging #98 into master will increase coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage    93.4%   93.77%   +0.37%     
==========================================
  Files           5        5              
  Lines         500      530      +30     
==========================================
+ Hits          467      497      +30     
  Misses         33       33

Impacted Files	Coverage Δ
src/LinearOperators.jl	93.5% <100%> (+0.7%)	⬆️

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[codecov]

Codecov Report

Merging #98 into master will increase coverage by 0.39%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   92.49%   92.89%   +0.39%     
==========================================
  Files           6        6              
  Lines         533      563      +30     
==========================================
+ Hits          493      523      +30     
  Misses         40       40

Impacted Files	Coverage Δ
src/LinearOperators.jl	94.42% <100%> (+0.65%)	⬆️

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[dpo]

Does @closure really help type stability??!?

[abelsiqueira]

According to @code_warntype, yes.

[abelsiqueira]

Closing in favor of #99