bv_pointerst: make all members return (optional) bvt

[tautschnig]

Instead of having in-out non-const bvt references, return bvt by value
and rely on copy elision for performance. This simplifies the code in
that it avoids creating (and sometimes unnecessarily resizing) objects
that are then to be populated by a function.

For convert_address_of_rec use optionalt to remove the true/false
error reporting.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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 #5831 (2d88418) into develop (09251eb) will decrease coverage by 0.00%.
The diff coverage is 73.93%.

@@             Coverage Diff             @@
##           develop    #5831      +/-   ##
===========================================
- Coverage    72.85%   72.84%   -0.01%     
===========================================
  Files         1423     1421       -2     
  Lines       154072   154138      +66     
===========================================
+ Hits        112244   112289      +45     
- Misses       41828    41849      +21     

Impacted Files	Coverage Δ
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%> (-4.12%)	⬇️
src/solvers/flattening/bv_pointers.cpp	80.89% <73.94%> (-1.53%)	⬇️
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%> (-0.23%)	⬇️
src/solvers/flattening/boolbv_width.h	100.00% <100.00%> (ø)
src/solvers/flattening/bv_pointers.h	100.00% <100.00%> (ø)
src/util/endianness_map.h	100.00% <0.00%> (ø)

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[martin-cs]

Seems reasonable, although I wonder if we should go further and do this throughout the solver. I suspect doing so will break patches against it though.