Move expr2bits, bits2expr, try_get_string_data_array to simplify_utils

[tautschnig]

They do not have any dependency on the simplify_exprt class (and thus
don't need to be class members). Similarly, try_get_string_data_array
has nothing to do with the simplify_exprt functions.

• 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.

[martin-cs]

Seems reasonable.

[codecov]

Codecov Report

Merging #5726 (3d817b2) into develop (0312102) will decrease coverage by 0.81%.
The diff coverage is 93.44%.

@@             Coverage Diff             @@
##           develop    #5726      +/-   ##
===========================================
- Coverage    69.55%   68.73%   -0.82%     
===========================================
  Files         1243     1243              
  Lines       100718   100721       +3     
===========================================
- Hits         70052    69233     -819     
- Misses       30666    31488     +822     

Flag	Coverage Δ
cproversmt2	?
regression	66.45% <77.04%> (+<0.01%)	⬆️
unit	32.22% <66.39%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/goto-symex/goto_symex.cpp	99.71% <ø> (ø)
src/util/simplify_expr_class.h	88.88% <ø> (ø)
src/util/simplify_expr_struct.cpp	69.56% <0.00%> (+0.33%)	⬆️
src/util/simplify_expr.cpp	86.34% <33.33%> (-1.41%)	⬇️
src/util/simplify_utils.cpp	98.54% <98.21%> (-1.46%)	⬇️
src/util/simplify_expr_int.cpp	79.80% <100.00%> (-0.28%)	⬇️
src/solvers/smt2/smt2irep.cpp	32.00% <0.00%> (-56.00%)	⬇️
src/solvers/smt2/smt2_conv.cpp	31.46% <0.00%> (-26.23%)	⬇️
src/solvers/floatbv/float_bv.cpp	55.25% <0.00%> (-18.88%)	⬇️
src/solvers/smt2/smt2_tokenizer.cpp	69.02% <0.00%> (-16.82%)	⬇️
... and 17 more

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