fix: redesign the core composition hirearchy

[hbagdi]

• introduce recursive composition of Unsplash type
  which holds the client
• remove the ugly type casting code from the entire library

[codecov]

Codecov Report

Merging #39 into master will decrease coverage by 0.46%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   79.84%   79.38%   -0.47%     
==========================================
  Files          17       17              
  Lines         893      873      -20     
==========================================
- Hits          713      693      -20     
  Misses        136      136              
  Partials       44       44

Impacted Files	Coverage Δ
unsplash/users_service.go	85.07% <100%> (-0.64%)	⬇️
unsplash/photos_service.go	85.13% <100%> (-0.68%)	⬇️
unsplash/collections_service.go	87.27% <100%> (-0.66%)	⬇️
unsplash/get_helpers.go	87.5% <100%> (ø)	⬆️
unsplash/search_service.go	89.28% <100%> (-0.55%)	⬇️
unsplash/unsplash.go	72.22% <92.85%> (-0.31%)	⬇️

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[coveralls]

Coverage decreased (-0.3%) to 81.843% when pulling 652c6a3 on fix/service-compostion into ba30b70 on master.