Functional Arguments for CreateContainer

[tri-adam]

Use functional arguments for CreateContainer. Define SpecVersion type. Switch several header const values to be non-exported.

Closes #28

[codecov-commenter]

Codecov Report

Merging #31 (36b1705) into master (c23314b) will decrease coverage by 0.27%.
The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   66.86%   66.58%   -0.28%     
==========================================
  Files          25       25              
  Lines        1672     1685      +13     
==========================================
+ Hits         1118     1122       +4     
- Misses        366      374       +8     
- Partials      188      189       +1     

Impacted Files	Coverage Δ
pkg/sif/lookup.go	65.00% <0.00%> (ø)
pkg/sif/load.go	53.22% <50.00%> (ø)
pkg/sif/create.go	50.00% <56.09%> (-0.72%)	⬇️
internal/app/siftool/modif.go	83.33% <100.00%> (+2.68%)	⬆️
pkg/sif/sif.go	28.23% <100.00%> (+1.72%)	⬆️

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