fix(int): resolve panic in unbounded_shr_vartime for zero shift

[fcasal]

Fixes #1212 by using unbounded_shl instead of shl.

Adds a few regression tests and a proptest.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.88%. Comparing base (236d679) to head (a97c1db).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
+ Coverage   87.86%   87.88%   +0.01%     
==========================================
  Files         184      184              
  Lines       21005    21028      +23     
==========================================
+ Hits        18457    18480      +23     
  Misses       2548     2548              

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[tarcieri]

cc @andrewwhitehead