Add initial ImageStorage trait.

[fabianfreyer]

Build an ImageStorage trait, and implement it for the Registry. Make everything else using the Registry directly reference this instead.

• Add the trait
• Implement for Repository.
• Change Image to use the trait, probably using a Box<dyn ImageStorage> inside, or something like it.

[codecov]

Codecov Report

Merging #15 into master will decrease coverage by 1.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage    86.3%   84.54%   -1.77%     
==========================================
  Files           6        6              
  Lines         336      343       +7     
==========================================
  Hits          290      290              
- Misses         46       53       +7

Impacted Files	Coverage Δ
src/image/go.rs	57.89% <0%> (-2.11%)	⬇️
src/image/manifest.rs	88.88% <0%> (-2.03%)	⬇️

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