Use std::from_chars instead of stoll/stoull in string2int.h

[kroening]

Summary

Replace the exception-based std::stoll/std::stoull implementation of string2optional<T> with std::from_chars (C++17). This removes three template functions (string2optional_base x2, wrap_string_conversion) and replaces them with a single, simpler template.

Benchmark Results

1.4M conversions (mix of valid/invalid inputs), -O2 clang++ on macOS:

Type	stoll/stoull	from_chars	Speedup
signed long long	1424 ms	8.9 ms	161x
unsigned long long	968 ms	10.2 ms	95x
int (with narrowing)	1446 ms	7.9 ms	184x
unsigned long long (base 16)	497 ms	5.7 ms	88x

The large speedup comes from: (1) no exception throw/catch on invalid inputs, (2) no locale overhead that stoll/stoull carry.

Semantic Changes

1. Leading whitespace: std::stoll/stoull silently skip leading whitespace; std::from_chars does not. In this codebase, inputs to string2optional are numeric literals and identifiers from the IR which never have leading whitespace, so this is not a behavioral change in practice.

2. Overflow handling: The old code converted via long long then narrowed (throwing out_of_range on overflow, caught by wrap_string_conversion). The new code uses from_chars directly with the target type T, which returns errc::result_out_of_range. The observable behavior (returning nullopt) is identical.

3. Partial parses: Both old and new reject strings that are not fully consumed (old via stoll semantics + narrow check, new via ptr != last check). The new version is actually stricter — stoll("123abc") would parse 123 successfully, while from_chars with the ptr != last check returns nullopt.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.56%. Comparing base (166a7d4) to head (9e2a3a7).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9007      +/-   ##
===========================================
+ Coverage    80.55%   80.56%   +0.01%     
===========================================
  Files         1707     1707              
  Lines       189016   189047      +31     
  Branches        73       73              
===========================================
+ Hits        152261   152308      +47     
+ Misses       36755    36739      -16     

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