Change to use Int instead of Natural for SomeBV creations

[lsrcz]

In #174, we introduced the unified SomeBV implementation and provided its creation with a run-time bit width. The run-time bit width was particularly hard to use in my experience.

This pull request changed it to use Int.

[codecov]

Codecov Report

Attention: Patch coverage is 15.38462% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 58.56%. Comparing base (f813166) to head (767d234).

Files	Patch %	Lines
src/Grisette/Core/Data/SomeBV.hs	15.38%	1 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
- Coverage   58.57%   58.56%   -0.02%     
==========================================
  Files          70       70              
  Lines        5813     5816       +3     
  Branches      485      486       +1     
==========================================
+ Hits         3405     3406       +1     
- Misses       1923     1924       +1     
- Partials      485      486       +1     

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