Optimize allocations

[PyDataBlog]

This PR closes:

• Switch from array indexing to byte indexing for n-grams generation. #20
• Optimise ngram generations to avoid unnecessary allocations #16

[codecov-commenter]

Codecov Report

Merging #21 (53d08d2) into main (f822944) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   99.40%   99.43%   +0.03%     
==========================================
  Files           5        5              
  Lines         168      177       +9     
==========================================
+ Hits          167      176       +9     
  Misses          1        1              

Impacted Files	Coverage Δ
src/dictdb.jl	100.00% <100.00%> (ø)
src/features.jl	98.33% <100.00%> (+0.29%)	⬆️

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