Updates to _mod_special operations for Uint, BoxedUint

[andrewwhitehead]

The tests for BoxedUint were not effectively testing different sizes, that is fixed now. BoxedUint also did not implement add_mod_special, which is added.

The tests for Uint were making extensive use of macros, this is updated to use const generics to simplify code generation.

[codecov]

Codecov Report

❌ Patch coverage is 98.92473% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.78%. Comparing base (decb7a1) to head (5d188b0).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/uint/boxed/add_mod.rs	97.50%	1 Missing ⚠️
src/uint/boxed/mul_mod.rs	98.00%	1 Missing ⚠️
src/uint/boxed/sub_mod.rs	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
+ Coverage   86.74%   86.78%   +0.03%     
==========================================
  Files         182      182              
  Lines       20384    20454      +70     
==========================================
+ Hits        17683    17750      +67     
- Misses       2701     2704       +3     

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