introduce cstrlen_exprt

[kroening]

This adds a class for the existing cstrlen expression, which denotes the length of a zero-terminated string that starts at the given address.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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 #7128 (4fe5b9d) into develop (731ff1f) will decrease coverage by 0.01%.
The diff coverage is 48.57%.

@@             Coverage Diff             @@
##           develop    #7128      +/-   ##
===========================================
- Coverage    77.88%   77.86%   -0.02%     
===========================================
  Files         1576     1576              
  Lines       181765   181804      +39     
===========================================
+ Hits        141567   141569       +2     
- Misses       40198    40235      +37     

Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	67.18% <ø> (ø)
src/cpp/cpp_declarator_converter.cpp	79.66% <0.00%> (ø)
src/cpp/cpp_typecheck_resolve.cpp	70.00% <0.00%> (ø)
src/cpp/template_map.cpp	57.85% <0.00%> (ø)
src/solvers/lowering/byte_operators.cpp	92.60% <ø> (ø)
src/solvers/prop/bdd_expr.cpp	93.02% <ø> (ø)
src/util/pointer_expr.h	80.83% <0.00%> (-5.43%)	⬇️
src/util/xml.cpp	73.88% <ø> (ø)
src/ansi-c/c_typecheck_expr.cpp	74.17% <7.27%> (-0.49%)	⬇️
src/solvers/flattening/bv_utils.cpp	78.33% <25.00%> (ø)
... and 43 more

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[jimgrundy]

What is the motivation for this change? What were we missing before?

[tautschnig]

What is the motivation for this change? What were we missing before?

Rather than providing a direct answer: see #7117 for some discussion.