replace libsodium with xsalsa20poly1305

[warner]

refs #58

[codecov-io]

Codecov Report

Merging #72 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #72      +/-   ##
=========================================
+ Coverage   50.44%   50.5%   +0.05%     
=========================================
  Files          24      24              
  Lines        1687    1689       +2     
=========================================
+ Hits          851     853       +2     
  Misses        836     836

Impacted Files	Coverage Δ
src/core/key.rs	43.39% <100%> (+1.08%)	⬆️

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