refactor Sauvola using the functor API

[johnnychen94]

This PR is done in the same manner as #42 except that:

I didn't move window_size out of Sauvola according to #41

[codecov]

Codecov Report

Merging #43 into api will increase coverage by 8.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              api      #43      +/-   ##
==========================================
+ Coverage   80.31%   88.94%   +8.62%     
==========================================
  Files          20       20              
  Lines         188      190       +2     
==========================================
+ Hits          151      169      +18     
+ Misses         37       21      -16

Impacted Files	Coverage Δ
src/sauvola.jl	94.73% <100%> (+94.73%)	⬆️

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