Incremental SMT2: Lazy array instantiation for array_of expressions

[tautschnig]

Replace the universal quantifier (forall) encoding for array_of_exprt with targeted index-specific assertions. Instead of asserting that every element equals the fill value upfront, track which indices are actually accessed and only assert the fill value for those specific indices.

This eliminates smt_forall_termt usage for array_of, improving SMT solver performance (especially with 64-bit indices) and avoiding potential 'unknown' results from solvers that struggle with quantifiers combined with arrays and bit vectors.

New data structures:

• array_of_fill_values: maps array identifier to its fill value term
• array_of_instantiated_indices: tracks which indices have been asserted

New methods:

• instantiate_array_of_for_index: sends a single select=fill assertion
• instantiate_lazy_array_of_indices: scans expressions for index_exprt nodes accessing array_of arrays and triggers lazy instantiation

The lazy instantiation is called from both set_to and ensure_handle_for_expr_defined after define_dependent_functions.

Fixes: #8061

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.42%. Comparing base (59d2211) to head (5dd6bb6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8899   +/-   ##
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  Coverage    80.41%   80.42%           
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  Files         1703     1703           
  Lines       188398   188489   +91     
  Branches        73       73           
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+ Hits        151502   151590   +88     
- Misses       36896    36899    +3     

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