Refactor: define loader as a singleton method

[qnighy]

Why

To support inheritance & mixin in the future, and just as a refactoring

What

Define a loader or a primary loader as a singleton method.

[codecov]

Codecov Report

Merging #21 (fb28349) into master (ae04bfd) will increase coverage by 0.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   91.81%   92.30%   +0.49%     
==========================================
  Files           3        3              
  Lines         171      169       -2     
  Branches       38       37       -1     
==========================================
- Hits          157      156       -1     
  Misses          7        7              
+ Partials        7        6       -1     

Impacted Files	Coverage Δ
lib/computed_model/dep_graph.rb	100.00% <100.00%> (ø)
lib/computed_model/model.rb	85.55% <100.00%> (+0.60%)	⬆️

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