Better support for string conversions

[konard]

Summary

This PR implements better support for string conversions as requested in issue #76. The implementation adds specialized converters for UTF string types while maintaining backward compatibility.

✨ Features Added

• Bidirectional conversion between std::u8string and std::string

  • Direct memory reinterpretation for efficient conversion
  • Assumes UTF-8 encoding for std::string (as per issue requirements)

• Chain conversions for UTF-16 and UTF-32 strings

  • std::u16string → std::u8string → std::string
  • std::u32string → std::u8string → std::string
  • Proper UTF-8 encoding for multi-byte characters

• Comprehensive UTF-8 encoding support

  • Handles 1-byte ASCII characters (0x00-0x7F)
  • Handles 2-byte characters (0x80-0x7FF)
  • Handles 3-byte characters (0x800-0xFFFF)
  • Handles 4-byte characters (0x10000-0x10FFFF) including emojis

🧪 Test Coverage

Added comprehensive test cases covering:

• Basic ASCII string conversions
• Unicode character conversions (accented characters)
• Emoji conversions (4-byte UTF-8)
• Bidirectional conversion verification
• Chain conversion validation

All tests pass successfully with the new implementation.

🔧 Implementation Details

• Template specializations: Following the existing code pattern and avoiding large function modifications as suggested in the issue comments
• Memory efficient: Direct reinterpretation for std::string ↔ std::u8string conversions
• Standards compliant: Proper UTF-8 encoding implementation for multi-byte sequences
• Chain pattern: Reuses conversion logic for maintainability

📋 Supported Conversions

Source	Target	Method
std::u8string	std::string	Direct memory reinterpretation
std::string	std::u8string	Direct memory reinterpretation
std::u16string	std::u8string	UTF-8 encoding
std::u32string	std::u8string	UTF-8 encoding
std::u16string	std::string	Chain: u16 → u8 → string
std::u32string	std::string	Chain: u32 → u8 → string

⚠️ Requirements

As specified in the issue, these conversions work correctly only when std::string is configured to use UTF-8 encoding by the compiler.

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Fixes #76

🤖 Generated with Claude Code