Int wrapping multiplication improvements

[andrewwhitehead]

This optimizes Int::wrapping_mul and Uint::checked_mul_int to make use of Uint::wrapping_mul, and adds Int::saturating_mul.

[codecov]

Codecov Report

❌ Patch coverage is 98.41270% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.84%. Comparing base (56e6c4a) to head (8b7d103).

Files with missing lines	Patch %	Lines
src/int/mul.rs	98.93%	1 Missing ⚠️
src/int/mul_uint.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #998      +/-   ##
==========================================
- Coverage   79.87%   79.84%   -0.03%     
==========================================
  Files         163      163              
  Lines       17593    17696     +103     
==========================================
+ Hits        14052    14129      +77     
- Misses       3541     3567      +26     

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