Introduce euclidean_mod expression

[kroening]

This introduces a variant of the modulo operator, to be used on integers,
which uses Boute's Euclidean definition. The result is never negative, and
thus differs from the C99 % operator. The semantics match what SMT-LIB2
does in the integer theory.

The operator % is not defined by SMT-LIB2, and is removed from the parser.

• 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 #6200 (2cf5418) into develop (0002950) will increase coverage by 7.85%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6200      +/-   ##
===========================================
+ Coverage    67.40%   75.26%   +7.85%     
===========================================
  Files         1157     1456     +299     
  Lines        95236   161169   +65933     
===========================================
+ Hits         64197   121297   +57100     
- Misses       31039    39872    +8833     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	?

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

Impacted Files	Coverage Δ
src/util/string_container.cpp	52.94% <0.00%> (-47.06%)	⬇️
src/solvers/prop/prop.cpp	42.85% <0.00%> (-41.76%)	⬇️
src/solvers/flattening/boolbv_member.cpp	53.65% <0.00%> (-38.65%)	⬇️
src/cpp/cpp_storage_spec.cpp	65.00% <0.00%> (-35.00%)	⬇️
src/util/cmdline.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/array_pool.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/string_refinement.h	66.66% <0.00%> (-33.34%)	⬇️
...rs/strings/string_concatenation_builtin_function.h	0.00% <0.00%> (-33.34%)	⬇️
src/cbmc/c_test_input_generator.cpp	60.00% <0.00%> (-30.33%)	⬇️
src/ansi-c/literals/convert_float_literal.cpp	64.28% <0.00%> (-27.61%)	⬇️
... and 1435 more

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[martin-cs]

A test case would be nice if possible.

[TGWDB]

Looks good, some test cases would be nice (particularly showing this difference on negative numbers).