Move crypto_box to the nacl-compat repo

[newpavlov]

Closes #338

[codecov-commenter]

Codecov Report

Merging #350 (d0d9156) into master (ff01185) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
- Coverage   89.40%   89.05%   -0.36%     
==========================================
  Files          42       40       -2     
  Lines        2020     1909     -111     
==========================================
- Hits         1806     1700     -106     
+ Misses        214      209       -5     

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