add two helpers for bv_typet

[kroening]

bv_typet is meant for bit-vectors without numerical interpretation, e.g., when constructing bit-masks. This adds two helper methods for creating a bv-typed constant with the frequently used bit patterns 0...0 and 1...1.

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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).
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• 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 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.36%. Comparing base (699331b) to head (977b3ee).
Report is 5 commits behind head on develop.

Files	Patch %	Lines
src/util/bitvector_types.cpp	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8350      +/-   ##
===========================================
+ Coverage    78.15%   78.36%   +0.20%     
===========================================
  Files         1726     1726              
  Lines       188869   188387     -482     
  Branches     18489    18445      -44     
===========================================
+ Hits        147614   147632      +18     
+ Misses       41255    40755     -500     

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