Remove boringssl dependency. Replace sha256 with absl::Hash.

[ivan444]

Closes #1962

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fa07295) 92.85% compared to head (5f238a6) 92.85%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1982   +/-   ##
=======================================
  Coverage   92.85%   92.85%           
=======================================
  Files         355      355           
  Lines       26272    26272           
=======================================
  Hits        24395    24395           
  Misses       1877     1877           

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[hzeller]

This will also fix issues found in chipsalliance/homebrew-verible#23

[hzeller]

Thanks, merged!