Do internal cleanup

[erni27]

This PR improves a few internal things in the codebase.

It contains no visible changes for the clients.

[codecov-commenter]

Codecov Report

Merging #35 (9305606) into master (e7deffb) will not change coverage.
The diff coverage is 97.50%.

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@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   94.94%   94.94%           
=======================================
  Files           9        8    -1     
  Lines         514      514           
=======================================
  Hits          488      488           
  Misses         22       22           
  Partials        4        4           

Impacted Files	Coverage Δ
eviction.go	71.11% <96.96%> (+71.11%)	⬆️
entry.go	100.00% <100.00%> (ø)
imcache.go	96.30% <100.00%> (ø)
option.go	100.00% <100.00%> (ø)

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