bitvector_typet: set width from mp_integer

[kroening]

This adds C-style getter/setter methods to bitvector_typet for the width of the vector.

The setter takes an mp_integer, which checks that the width fits within std::size_t. This removes the burden of performing this check from the caller, and allows callers to avoid potential arithmetic overflows by using mp_integer.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.38%. Comparing base (4bd5c0a) to head (0079017).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_types.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8477      +/-   ##
===========================================
+ Coverage    78.23%   78.38%   +0.15%     
===========================================
  Files         1727     1727              
  Lines       189050   188685     -365     
  Branches     18467    18475       +8     
===========================================
+ Hits        147898   147900       +2     
+ Misses       41152    40785     -367     

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