Refactor to switch-case; decrease indentation

[alexandear]

This PR does refactorings:

• change if-else if-else with switch-case;
• change switch with single case to if;
• decrease indentation by using continue.

[codecov]

Codecov Report

Merging #122 (56f395a) into master (0d5a4c9) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   90.08%   90.06%   -0.02%     
==========================================
  Files           8        8              
  Lines         625      624       -1     
==========================================
- Hits          563      562       -1     
  Misses         43       43              
  Partials       19       19              

Flag	Coverage Δ
unittests	90.06% <100.00%> (-0.02%)	⬇️

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Files Changed	Coverage Δ
reviser/file.go	94.79% <100.00%> (-0.02%)	⬇️

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