bv_pointerst: encapsulate offset/object bit layout

[tautschnig]

Hide erase/resize operations behind a proper interface.

• Each commit message has a non-empty body, explaining why the change was made.
• 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

Merging #5832 (ad7584b) into develop (81ecf05) will increase coverage by 0.01%.
The diff coverage is 82.00%.

@@             Coverage Diff             @@
##           develop    #5832      +/-   ##
===========================================
+ Coverage    72.84%   72.86%   +0.01%     
===========================================
  Files         1421     1421              
  Lines       154138   154173      +35     
===========================================
+ Hits        112289   112337      +48     
+ Misses       41849    41836      -13     

Impacted Files	Coverage Δ
src/goto-programs/goto_convert_class.h	87.30% <ø> (ø)
src/solvers/flattening/boolbv_union.cpp	57.14% <0.00%> (ø)
src/solvers/flattening/boolbv_with.cpp	65.30% <0.00%> (ø)
src/solvers/flattening/boolbv.h	69.56% <50.00%> (ø)
src/solvers/flattening/bv_pointers.cpp	81.18% <77.07%> (+0.28%)	⬆️
src/goto-programs/builtin_functions.cpp	54.57% <94.11%> (+1.03%)	⬆️
src/ansi-c/c_typecheck_expr.cpp	74.42% <100.00%> (+0.40%)	⬆️
src/solvers/flattening/boolbv_byte_extract.cpp	70.00% <100.00%> (ø)
src/solvers/flattening/boolbv_byte_update.cpp	100.00% <100.00%> (ø)
src/solvers/flattening/boolbv_width.cpp	76.69% <100.00%> (ø)
... and 16 more

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[martin-cs]

Thanks for tidying this up.

[martin-cs]

If you are making changes to this PR some documentation here might be nice.

[tautschnig]

I arguably should have done this right away. Now fixed.