Reapply "Write random_mod in terms of new random_bits"

[mrdomino]

Depends on RustCrypto/signatures#1139 as well as potentially changes to other dependent crates.

Reverts #1060

[codecov]

Codecov Report

❌ Patch coverage is 84.05797% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.70%. Comparing base (b3a1ae7) to head (dd0c726).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/uint/boxed/encoding.rs	50.00%	7 Missing ⚠️
src/uint/rand.rs	94.23%	3 Missing ⚠️
src/uint/boxed/rand.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
- Coverage   78.77%   78.70%   -0.07%     
==========================================
  Files         173      173              
  Lines       17741    17756      +15     
==========================================
  Hits        13975    13975              
- Misses       3766     3781      +15     

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