Add irept::set_size_t to avoid unnecessary casts

[tautschnig]

The integer-typed argument to irept::set is converted to a string, which
works equally well for types other than long long. Naming the new method
just "set" would result in ambiguous overload, thus using set_size_t
(which is also consistent with the existing get_size_t).

• 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 #6052 (ae1aac1) into develop (ff5902e) will decrease coverage by 0.44%.
The diff coverage is 89.83%.

@@             Coverage Diff             @@
##           develop    #6052      +/-   ##
===========================================
- Coverage    74.25%   73.81%   -0.45%     
===========================================
  Files         1444     1444              
  Lines       157460   157448      -12     
===========================================
- Hits        116928   116220     -708     
- Misses       40532    41228     +696     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.h	94.11% <ø> (ø)
src/solvers/flattening/boolbv.cpp	80.00% <0.00%> (+0.47%)	⬆️
src/solvers/flattening/boolbv_case.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/boolbv_cond.cpp	0.00% <0.00%> (ø)
src/solvers/prop/prop.cpp	42.85% <0.00%> (-42.86%)	⬇️
src/solvers/prop/prop.h	84.37% <ø> (ø)
src/solvers/sat/cnf.h	83.33% <ø> (ø)
src/util/irep.h	93.47% <ø> (ø)
src/util/std_expr.h	92.08% <ø> (-0.56%)	⬇️
src/goto-instrument/code_contracts.cpp	84.75% <100.00%> (+0.46%)	⬆️
... and 60 more

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