mgm: use parallel block encryption if possible

[newpavlov]

No description provided.

[codecov-commenter]

Codecov Report

Merging #358 (88a464f) into master (e470a51) will increase coverage by 0.22%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   89.24%   89.47%   +0.22%     
==========================================
  Files          39       39              
  Lines        1888     1948      +60     
==========================================
+ Hits         1685     1743      +58     
- Misses        203      205       +2     

Impacted Files	Coverage Δ
mgm/src/lib.rs	98.68% <97.22%> (-1.32%)	⬇️

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