Disambiguate overloaded insert(...) calls [blocks: #6749]

[tautschnig]

Once we move to std::optional, the use of int results in an ambiguous
overload resolution. Avoid that by explicitly constructing an
mp_integer.

• 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 #6755 (6aca25c) into develop (c0f179c) will increase coverage by 0.02%.
The diff coverage is 84.55%.

@@             Coverage Diff             @@
##           develop    #6755      +/-   ##
===========================================
+ Coverage    76.87%   76.89%   +0.02%     
===========================================
  Files         1590     1590              
  Lines       183887   183978      +91     
===========================================
+ Hits        141364   141477     +113     
+ Misses       42523    42501      -22     

Impacted Files	Coverage Δ
src/ansi-c/goto_check_c.cpp	91.47% <ø> (-0.15%)	⬇️
src/pointer-analysis/value_set_fi.cpp	69.78% <50.00%> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	72.09% <79.48%> (+3.07%)	⬆️
src/pointer-analysis/value_set.cpp	83.33% <83.33%> (+0.02%)	⬆️
src/analyses/goto_rw.cpp	62.83% <100.00%> (+0.47%)	⬆️
src/goto-programs/builtin_functions.cpp	58.35% <100.00%> (+1.48%)	⬆️
src/util/pointer_expr.h	80.68% <100.00%> (ø)
src/util/simplify_expr_int.cpp	85.59% <100.00%> (ø)
src/util/std_expr.h	93.66% <0.00%> (+0.18%)	⬆️
... and 5 more

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