[stdlib] Use memcmp to dispatch to optimized implementation in StringSlice

[YichengDWu]

This PR updates the comparison logic in StringSlice to use the public memcmp API instead of the internal _memcmp_impl_unconstrained.

Previously, StringSlice was directly calling _memcmp_impl_unconstrained. By switching to the standard memcmp, we allow the call to be properly dispatched to the optimized implementation (_memcmp_impl_opt_unconstrained).

This ensures that string slice comparisons leverage the best available memory comparison implementation provided by the standard library.

[Copilot]

Pull request overview

This PR modernizes the StringSlice comparison logic by replacing a direct call to the internal _memcmp_impl_unconstrained function with the public memcmp API, enabling automatic dispatch to optimized memory comparison implementations.

• Removed direct import of internal _memcmp_impl_unconstrained function
• Updated __lt__ method to use public memcmp API instead

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[JoeLoser]

!sync

[modularbot]

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[modularbot]

Landed in 3fe4f86! Thank you for your contribution 🎉